t1. It is absolutely possible. Like, I-do-not-comprehend-the-question possible.
2. MCAT is king. Do not, under any circumstances, mess up your MCAT.
3. Honestly, I think it's a good idea to have 1-2 'off' days per week while study for the MCAT anyway. Even the intense SN2ed schedule had 1/wk, but I took more (was scribing p/t and working in biotech p/t), and with the extra content to cover now, aka more extended study period, I think it's even more important to pace yourself.
4. If you think you can do literally anything and study for the MCAT, I'd highly recommend scribing as that anything. So much valuable experience. It was a game-changer for me, perspective-wise. It's the only pre-med activity where I actively felt as if I were learning and contributing at all times (even beyond the hands-on volunteering where I actively performed a screening test on patients and recorded the results in the chart). It was where I decided whether the weird, long, hours and the hospital BS and often missing your lunch breaks or barely getting to pee on a shift were things I could live with. In fact, I found that I wouldn't even notice those details when I was amped up by a heavy patient load or an intense case. So, ymmv, but I think it's one of the best things you can do premed.
So, yeah. It shouldn't make a difference. Most people can do both. If you can't, absolutely pick the MCAT, but I'd also pay a lot of attention to how you study and how effective your methods are for your own future benefit. This will not be your last standardized test, by any stretch - and you won't be able to take time off for all of them. Also be careful not to burn out studying 7d/wk nonstop!