Hmmm… here’s the thing.
YES studies point to equated weekly volume leading to similar gains (in that case, they typically look at 1 vs 2 or 3 weekly sessions with the same total weekly volume). But there is still a training stress threshold to reach at each session to trigger growth.
It’s not like on Sunday you body looks at the volume it did for a muscle and boom make growth happens.
Each sessions stimulates growth on its own.
If you have one large session for a muscle you get a larger (relatively speaking) amount of growth after that one workout.
If you have several small sessions for a muscle, after each session you get a smaller amount of growth, but if you add up the growth from all these sessions it can add-up to the same amount as that one large session did.
BUT there is a minimal dose of exercise-induced stimulus that you must reach in any given session to trigger the growth processes.
If you fail to reach that threshold, that session does not stimulate any growth.
Even if you have a lot of these small sessions throughout the week it still amounts to no growth because it’s a case of 0 + 0 + 0 + 0 + 0 = 0
My point is that, yes, it is fine to spread the biceps/back volume into smaller pieces and put them at the end of the other workouts. BUT if the pieces are too small, it just won’t work.
I’ll use an extreme example, if you do 1 set of biceps and 1 set of back 5 times a week, it is likely not going to be very effective, in fact it will likely not lead to any growth: the daily stimulus for those muscle simply not being high enough.
IMHO you’d need at least 3 work sets for a muscle (unless you are a beginner) to give a strong enough growth signal to make it work.