As a writer, I am sticking with a single couple focus.
As a reader, I've seen many authors handle the dual romance, with a primary or alpha couple, and then the secondary beta couple. More word count is devoted to the alphas, but the betas figure key into the story. I see this more in things like single title historicals. I also think it's fading out in lieu of "seeding for a series". Now I see secondary characters introduced and then followed up in the next or the next book as the lead. I don't think all secondary characters really warrent a book, but I understand the love of series in readers and the marketing strategy.
If you're working with a category romance, it will be very difficult to add in a second couple romantic plot, but I've seen it done. It's with a light and sparing hand.
Does your story require you have two romances at once? If not, and this is your first walk in this genre, I'd focus on the alpha couple. However, if that second couple and thier romance (each Goal/motivation/ & conflict) is critical and central to the story, I'd write it out to see what happens, and be careful to ask myself: if they're that critical, why are they secondary characters?
BTW, you might want to check out
http://www.eharlequin.com
They have good links, and, info on writing in the romance genre.
Also, check out:
http://karenfox.com
She has some awesome info such as what houses average what advances, as well as some slamming links.
Last, look into the Romnace Writers of America organization.
Good Luck.
Ursula
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