Increasing training frequency from once to twice per week, while equating total training volume, produces a statistically negligible effect on muscle hypertrophy.

Study: Equal-Volume Strength Training With Different Training Frequencies Induces Similar Muscle Hypertrophy and Strength Improvement in Trained Participants

When people do the same total amount of lifting each week, whether they do it in two sessions or four, their muscles grow about the same. So, splitting workouts into more days doesn’t help you get bigger if you’re lifting the same total weight.

Study: The Resistance Training Dose-Response: Meta-Regressions Exploring the Effects of Weekly Volume and Frequency on Muscle Hypertrophy and Strength Gain

When you do the same total amount of lifting each week, doing it in two sessions instead of one doesn’t make your muscles grow any bigger — the study found no meaningful difference.

Study: One, two, or three times a week? examining the optimal frequency for strength and muscle growth in accentuated eccentric exercise

When people lift weights the same total amount each week, it doesn’t matter much if they do it in one session, two sessions, or three — their muscles grow about the same. So going from once to twice a week doesn’t give you extra muscle growth if you’re doing the same total work.

Study: A meta-regression of the effects of resistance training frequency on muscular strength and hypertrophy in adults over 60 years of age

When older adults lift weights the same total amount each week, it doesn’t matter if they do it in one long session or split into two shorter ones — they gain about the same amount of muscle.