English doesn't have any "grammatically valid" construction for this particular conjunction.
Nor would it be particularly useful if there was such a construction. Partly because it would rarely be appropriate, but mainly because OP wondered where to leave that or who to leave that with. The only connection between those two possibilities are the repeated words leave, to and that (which should be "it" anyway). It simply isn't worth trying to fit both possibilities into a single construction to save a couple of words.
Also note that OP's suggestion already uses "or" to conjoin the first two variants (where, who), but a slash to conjoin to, with. At best this would be inconsistent use of "pseudo-syntax" — but in this case it makes no sense, since you can't select "to" from the second pair to create a valid expression.
In such circumstances, normal practice would be to either completely rephrase and state both possibilities fully and grammatically...
I was wondering where to leave it, or who to leave it with. 
...or (more common in speech) to simply enforce grammaticality for the second possibility only...
I was wondering where or who to leave it with.
 Per comments below, "who to leave it to" is grammatically valid, but inappropriate in this case, since to "leave [something] to [someone]" means to transfer ownership - normally in the context of a will or bequest, to be actioned after one's death.