I see it the exact opposite. If Rudy does not win Florida, that pretty much ends his campaign, reverting that all to McCain. McCain, in that case will win NY, NJ, and maybe California. Those are all super delegate states that will propel him to the front. He will be the defacto front runner. McCain and Rudy share many of the same voters, they can not both be viable, and I think vs Mitt, McCain has a much stronger advantage than Rudy does.
If Rudy does not win, I think it is over for everyone but McCain.
I agree, it’s hard to see Rudy voters going to Romney, but OTOH, I am not sure Rudy will drop out if he loses FL (which I expect him to do). He’s stubborn, so he might hang in through Super Tuesday.