Ummm.... I really don't see where this 'each write writes one romance' thing is coming from? DaGaider wrote two (even though he did pick one up). In ME3, Patrick Weekes wrote Jack, Tali, Kasumi, Mordin, and the Tuchanka and Rannoch arcs... I'm sure he could handle writing two romances 
Edit: And I like to think that PW tweet where he's going on about the complications of writing romance stuff comes out of the fact that he got stuck with writing two 
It's mainly because if Hepler was still around, she would have been writing Cassandra (in fact she apparently already had finished writing her and her romance, and Gaider had to rewrite her, if what I read is correct) which would have meant Gaider would have had only Dorian.
As of right now:
(For romances)
David Gaider - Cassandra, Dorian
Lukas Kristjansen - Sera
Sylvia Feketekuty - Josephine
Patrick Weekes - Iron Bull
Brianne Battye - Cullen
if going by that logic, Vivienne could be Mary Kirby's romance, and Sheryl Chee could be writing Blackwall's romance. Sheryl's also written a LI in both DA:O and DA2, so who knows.
Anyways, it's basically looking into things A LOT, and honestly at the moment it doesn't mean anything as there really isn't enough evidence to support the theory, but what's where people are coming from that are worried about the "one romance per writer" thing. I'm not all that worried about it, but it does sit in the back of my mind, I admit.