I only associate Quinn with girls, but will admit it would be fine for a boy too. DH LOVES this name and I actually love it too, but I am torn on the whole gender association thing and other people not knowing she's a girl by seeing her name on a list - that bothers me! Yep - 37 weeks pg and still not set on a girl's name yet (we are team green).
I think Quinn Linda is fine...it's not the best flowing name I've ever heard but certainly not the worst and IMO it's a rare occasion when people actually address someone by their fn and mn at the same time. I agree with pp's - in some cases family value is more important than flow.
I'm not sure many names will flow well with Linda as a mn. If you don't LOVE the name Quinn though, I'm not sure you should use it.