A few things about the depiction of True Love in season 1 made me think of it as more like magic generated by strong love than a one-off soulmate connection - for example, Charming realising that it was the gold transformation preventing Abigail from releasing Frederick instead of considering that they might not be True Love, and Regina's assumption that Belle could use a TLK to break Rumple's Dark One Curse. Then the revelation that familial love also counts made me think of scenarios where a person has multiple children, as well as a child having two or more parent figures.

All of this made me wonder if you could potentially have multiple romantic True Loves too. In The Stable Boy, young Regina did describe her relationship with Daniel as true love, but we didn't see any magical proof of that claim. The closest thing we had was Graham beginning to remember his real life after kissing Emma. But then that was a sort of halfway TLK, because he didn't remember fully and the Curse itself wasn't broken.

I don't think a polyamorous True Love situation would happen though - not in story. That'd be too controversial :P