I'm not so sure. I think what works for Romney is to try to destroy his opponent with negative ads and coordinated negative attacks in the press.
Whoever your preferred candidate is, at least until we get into winner-take-all primaries, I think the case can be made that it makes Romney less effective because he's having to attack on two fronts... plus it allows one "not-Romney" to attack Romney leaving the other immune to the charge of being "negative".
I agree with you. Newt and Rick are bleeding Romney of cash by both staying in. I think they can take him down if they tag team him.