Going half Catalina. What could go wrong? :)

[vdubreeze]

I've been happily chugging along with 10.11.6 El Capitan on two MBPs, a mid 2009 13" and mid 2015 15". I've finally hit a wall of sorts where some non-music software won't run on it. My plan is to put Catalina on the 15" and leave the 13" alone. Reaper works well on both as the are, usual Mac caveats apply.

My only concern is moving to APFS on the updated one. I'm not clear on compatibility of external drives used on both, and Google is not being my friend here. I sneakernet a lot between the two, using the 15" where processing power makes a difference, and the 13" for editing, small projects, etc. and in moving drives between the two don't want to give up that going at least in one direction a MBP can read a drive with files just copied from the other MBP. I'm assuming 10.11.6 can't read APFS, but what else happens?
Does Catalina only force APFS on the system drive and not attached drives? Anyone else keep two laptops running with a new APFS system and an older Mac OS Extended file system that share drives?

Thanks

[cyrano]

You only need APFS for the startup drive with Catalina. External disks should still be exFAT or HFS+, imho.

AFAICT there are no problems with HFS+ data drives under Catalina.

[vdubreeze]

Thanks, cyrano!