It might be time for a little check-in with the FWB partner who is acting more attached to you. Perhaps you could ask at a non-sexual moment how they were doing, if anything was bothering them, and if their feelings had changed about you.
If it seems difficult to bring up out of the blue, then maybe you could ask in response to some "attached" action. Is the person asking where you're going all the time? Are they wanting to spend more time with you aside from the benefits stuff? Are they asking what your favorite flower is? If they do something that makes you wonder if they're more attached, maybe you could say, "We didn't used to do this before (or "I don't remember you asking about that before..."), and that makes me wonder if things have changed between us. Have they?"
It's direct, but you can say it in a kind way like a friend would. I think it's better to be direct about those kinds of issues so everyone can stay on the same page.