I've never kept them together. I have been warned by other seahorse keepers that seahorses and clams don't mix. One person I know had to rescue a seahorse from INSIDE a clam, only the head and tail were showing, the entire body was clamped inside. I'd be afraid of hurting the seahorse AND the clam trying to separtate them in a situation like that.
Yes, seahorses and clams have different needs. You can keep asking around, to see if anyone has had experience keeping both together, maybe they'll have a more specific answer as to how it can be done, but I haven't heard of a successful tank with both.
Try a clam forum, maybe. Either way, if enough people say it can't be done, maybe you should take their word for it. If you still want to risk it, please report on how it goes with your tank so we can revise or confirm our advice.