So, the way I understand it... use a skip. Unlock the next map/level. Now use a multi... work on them both at the same time. So, started the highest level I reach. Check. Both items used, next level unlocked. Check. And... somehow I'm working on two instances of the same map simultaneously.

Ok, I have to assume it's my fault. I get to race myself. Wee!

Here's a question. Should it even be possible? Sure a multi without a skip can't get you the desired effect either. But a player can still use a multi by itself to complete achievements in a previous map while working on their highest current level. I can't see any benefit to my boneheaded mistake of using the multi to play the same level simultaneously.

So, should it even be possible? Would it be wrong to prevent players from doing this unintentionally OR intentionally? I just can't figure out a reason that isn't silly.

Yes, it should be possible. For example to grind last map simultaneously for greater number of APs than grinding two different levels. You used multilevel, you will get reward twice from that map.