Med students often work with multiple PIs even within the same department. The only time this becomes an issue is if you overextend yourself and start not making good on commitments to either PI - not a great look. This assumes that you're talking about clinical projects. If you're talking about basic science, it's very hard to do multiple basic science projects when they're across different labs (need to dedicate time in each lab for basic science projects so it's easier to just work on multiple projects with the same PI).