Can You Make Gains With Only 2 Workouts Per Week?

For most typical lifters, 3-4 weight training workouts per week is the benchmark when it comes to increasing muscle size and strength.
This is the "sweet spot" that I recommend to most trainees since it's a high enough frequency to optimize the overall effort for each muscle group without going overboard.
However, I was recently posed with an interesting situation. A man working 50-hour weeks, jui jitsu trains 3 times per week, runs a household, wants to know if he could make some solid gains with just training 2 times per week. 
Although 3-4 weekly workouts are usually the best way to maximize growth (5 can work in some circumstances, but only for more accomplished lifters), can twice-weekly training still generate good results?
Let's discuss…
Yes, working out twice a week can help you increase muscle mass and strength over time, as long as your workouts are properly structured and performed.
Is doing out twice a week the best way to get the best physique in the least amount of time? Probably not, but keep in mind that "ideal" and "effective" are two different things. Just because a particular training approach isn't the greatest way to go about things doesn't mean you can't make substantial improvement with it.
As long as you stick to a workout plan that directly trains each muscle group at least once per week with sufficient volume and intensity and allows for progressive overload, you'll see consistent results.
In reality, during my early bodybuilding days, when I used a low volume/high intensity strategy to lifting (often known as "HIT," or "high intensity training"), I went through times when I only went to the gym once every 3-4 days.
I was able to make consistent gains from session to session while only doing two weight training workouts per week on average. Could I have progressed more quickly if I had used a higher frequency approach? Probably, but the point is that lower-frequency training is still effective.
In past years, hitting each muscle group merely once a week was a conventional bodybuilding recommendation, and even on a two-day-per-week training schedule, this may be accomplished with adequate volume and intensity.
However, if you actually have the time and energy for three weekly sessions, this should not be used as an excuse to train less frequently. Remember that you don't need to spend hours upon hours in the gym to have a good exercise – it usually only takes 60-75 minutes from start to finish, including your warmup.
So, before you go any further, I'd advise you to take a close look at your weekly schedule to see where you might be able to squeeze in that one more training session. When you break things down piece by piece, you'll frequently find that there are a plethora of options.
If you don't have the time or energy to perform more than two exercises a week (or if bodybuilding is more of a hobby for you than a primary focus), here's how I'd go about it...
You'll want to keep things simple and to-the-point if you're just going to the gym two days a week. A full-body workout is an excellent approach to do this. You don't have to do a ton of different workouts or worry about hitting each muscle from every angle possible.
Stick to the fundamentals and focus your sessions on the large complex exercises that target your greatest muscle groups, with the goal of achieving consistent strength improvements over time.
These are the main movement patterns you should focus on:
Squat (barbell back squat or any other basic squatting variation)
Hip hinge (deadlift, Romanian deadlift, glute-ham raise etc.)
Horizontal press (bench press, dumbbell press, machine press etc.)
Horizontal pull (barbell row, dumbbell row, machine row etc.)
Vertical press (military press, seated overhead dumbbell press etc.)
Vertical pull (pull up, chin up, pulldown etc.)
Rather than attempting to incorporate all of these exercises into each workout, it's a good idea to split them up and do half on one day and half on the next.
For example, for workout A, you could do a squat, horizontal press, and horizontal pull, and then for workout B, you could do a hip hinge, vertical press, and vertical pull. After you've completed the main compound lifts, add in 2-3 accessory movements to target minor muscular groups like the biceps, triceps, side/rear delts, calves, and abs.
An upper body/lower body split (very self explanatory) or a push/pull split (squat/horizontal press/vertical press one day, hip hinge/horizontal pull/vertical pull the next) are two more choices for planning out an effective twice-week routine. Any of these ways will work in the end, and you can try them all out to determine which one you like.
Keep in mind that, in comparison to 3-4 sessions per week, you'll have a reduced "margin for error" because you'll be training with fewer frequency and volume.
To put it another way, you need to make sure you're getting the most out of each session by training at a high enough intensity and performing each set with appropriate technique. I wouldn't recommend training any less than 1 rep short of concentric muscular failure to maximum muscle stimulation.
If you're a little more accomplished and want to condense your workouts even more, you can drop the volume (2 sets per exercise, for example) and execute an HIT-style routine by going all the way to muscular failure on every session.
Make sure to write down your workouts in detail, including which exercises you did, how much weight you lifted, and how many reps you did on each set, just like you would with any other muscle-building workout regimen.
Then, each week, try to improve by doing an extra rep or two with the same weight, or increasing the load by 5-10 pounds. You'll know your twice-weekly training schedule is working if you're progressively adding more weight to the bar over time and your nutrition is on order.
On the subject of diet, bear in mind that if you're not training as frequently during the week, you'll want to reduce your overall calorie consumption significantly. You won't need as large a calorie surplus to maximize your gains because you'll be burning fewer overall calories than if you trained 3-4 days a week or more (assuming you're not very active outside of weight training). When you just exercise twice a week and combine that with an abnormally high calorie diet, you're more likely to acquire some unwanted fat.
While studies suggests that somewhat higher frequency sessions are best for promoting hypertrophy in most average trainees, two workouts per week might still be helpful.
Perhaps you have a hectic schedule and struggle to find the time and energy to exercise more frequently... Perhaps you have other active activities, such as athletics or martial arts... or perhaps bodybuilding isn't your top priority right now and you have other things to worry about.
If that's the case, compressing your workouts (at least initially) is usually the preferable option, since it increases your chances of sticking to your training plan in the long run. After all, a somewhat sub-optimal fitness plan that is continuously followed is significantly superior to an optimal but unsustainable workout routine.
Many people have achieved fantastic physiques in this manner, and you may as well if you plan your exercises properly, train hard, and eat appropriately.