Just curious. Has anyone shot with both systems at the same time, ie one bow with split and another bow with three under?

no but Ive shot one bow both ways.
Both have their pros and cons.
Depends on the archer.

I have a buddy that does very well.  As for me, I am pretty simple and I need to keep things like sight picture pretty simple so I stick with one bow, one arrow, one crawl---if not when its go time I will make mistakes.

If I were playing a longer distance game then I would go to split finger, but most of my shooting is inside 30 yards so its 3 under for me.

Yes, I shoot 3-D and hunting bows split and my indoor bow 3 under. No problems at all.

Yes. I used to shoot closer shots 3 under and longer shots split even shooting the same bow. No different than what they call a fixed crawl today. I thought I was really onto something until I started researching and found out it is pretty old. If you are alternating bows you really aren't changing the fundamentals.

Thanx for the replies. I try to keep my bows shooting at roughly the same speed so the trajectory for all of them is similar. But I have one that has a really flat trajectory and I might go to three under for that one.

Yes I do.  Shooting split since 1962, I did switch to three under in the early '60's for the short targets for field archery for a short time as most folks I shot with did.  Even changed anchor points for the 80 walk up target, which was allowed then.  That past experience got me comfortable with switching for different bows on my rack now, which are mostly a bunch of old rosewood Bears of that same era.   

There can be a difference among my bows.   A 2013 59'er, the way I have it set up (brace, nocking point, string type, etc), it shoots more quietly and smoothly shooting split than three under, but my other old 1959 to '62 rosewood Bears shoot about the same either split or three under.    I could adjust the 59'er to be better three under with a nocking point or brace adjustment, but like it just as it is, which is super quiet compared to those old rosewoods.  Also shoot an old 52-incher split.

I shoot 3 under and a fixed crawl. I don't shoot split anymore. >>>-----> Ken