Different genre tie-in – good idea or not?

So, I'm 61K into "Under the Hill", and I'm now completely sure that I have a Fantasy novel on my hands, not a romance.  This is all well and good, except that two of the characters (George and Chris) have backstory together – massive, massive backstory.  I find myself really wanting to write their massive backstory as a m/m romance novel.

The trouble is – can I write a Fantasy, sell it to a Fantasy publisher, and then use two of the characters in that book for a Romance, which I would...