The way I look at it, with flute and now clarinet, I practice what I need to on flute e.g. scales, long notes, technique, etc during one session e.g. hour or so, take a break for something to eat for example, to give my embouchure time to relax, then go back to practice on Clarinet doing similar but practicing pieces or sight reading instead, or more thoroughly than scales for example and vice versa for the next day. Although muscle memory is building up separately for each instrument the commonalities of techniques e.g. remembering actual notes names for scales, reading ahead & playing in correct key etc for sight reading are being built up whichever instrument I concentrate on.
I think now being an adult learner returning to music after a break of more than 15 years since school, I've found any time you can spend of 15 mins or more per day per instrument will show improvement & the longer the better, but as mentioned previously in other threads, sometimes taking a break for a day on a particular instrument will make you appreciate how
much you've improved when you go back to it rather than feeling that you have to practice for hours to make any progress .I've noticed I'm less likely to make progress practicing too long, because I get tired and make more mistakes.
Also, as was recently pointed out to me on something unrelated "there are two 8 o'clocks in the day" as I always prefer my lie-ins

, so you could always practice in the morning before school/work for one instrument (or half of however many you play) and then practice in the evening on the others. This may even show better progress because you've time to reflect throughout the day on areas you need to practice from your morning session!