Should I choose the same editor for two papers?
I've have multiple papers under review at the same time in the same journal for various reasons (e.g., special issues, highly preferred venue, standard community venue).


When the papers were linked, we made sure to request the same editor---otherwise, you're just making life more difficult for the editors because the editors of the two papers will have to coordinate in any case.
When the papers were not linked (and I've had utterly unrelated papers simultaneously under review, since they came from different collaborations), we selected an appropriate editor to request independently, which might or might not turn out to be the same.


The key principle here is to remember that your selection of an editor is only your personal recommendation to the journal.  Just like a statement preferred reviewers, the journal has no particular compulsion to abide by your selection. If they think a different editorial distribution makes sense, they will assign it that way instead.  

Thus: don't handicap a paper unnecessarily by recommending a suboptimal choice of editor.