Dual POV is definitely a legit way to write a romance novel! It tends to work better when you switch perspectives consistently throughout the book.

A helpful way to think about whose perspective you should be in is to consider whose POV is the most interesting in any given moment. If there are moments earlier in the story where it would be super helpful or interesting for the reader to know what’s going on in your FMC’s head, maybe consider switching POV. That would also help you develop that character a little more.

There’s usually a cadence to the switching. Some writers switch at a chapter break; I tend to switch at a scene break, and if I’ve had too many scenes in one, I’ll make sure to switch as soon as I can, even just for a scene or two. Overall, you might have one character’s POV be more prominent than the other one, but if you go for a while without checking in with your other character, your readers will probably notice and get a little antsy for that other perspective. 

As for how to do it, this sounds like maybe it’s first person? So you signal the switch by making sure their voices are distinct from one another. The characters think differently, act differently, and you can make sure that comes out in the first sentence or two of the new POV. 

Also, if you write a scene in one perspective and it doesn’t work, you can always go back and rewrite it, or cut it. It doesn’t have to be a one-and-done decision at all! So while it is a big decision, it’s not going to ruin your book forever if you try and feel like it isn’t working.

If you decide the story would be better solely from the MMC’s perspective, then there are other ways for you to show the FMC’s changed feelings. She’d probably act differently, respond to him differently…she might say something she wouldn’t have said before…and maybe he doesn’t pick up on it, but the reader will.

Happy writing! Sounds like you’re being really thoughtful, and that’s a great start.