If one has an interest in something and a fascination with something which one of the next is correct?
I've always had an interest and fascination in something
I've always had an interest and fascination with something
OP's usage is thus never going to sound quite right, because he's trying to force a single preposition to fit two mutually incompatible contexts. Strictly speaking, it should be...
But actually we often avoid that complexity by just letting the preposition agree with the final context, so in principle OP's usage should be okay using with. The problem here is that in practice, to have a fascination with something is nowhere near as common as to have an interest in it, so using with to cover both contexts still doesn't sound too good.
Therefore I think the best solution is to swap the contexts around...
I've always had an interest, and fascination, in something.
I find that focuses the attention on interest, making the answer fairly straightforward. Of course, you could switch the order, as long as you switch the corresponding parts too.