The only good reason for 'dual antennas' is that they are directional. That means that signals will 'improve' in two directions and get worse in the other two directions. If that suits your purpose than dual antennas (phased) should work for you.
I agree, where you propose mounting them, bumper, is one of the worse places you could pick. Bexause of that positioning the resulting radiation pattern will be very unusual, not what you'd expect at all. On the bed rails would be a 'better' place to put them, but that'd also me sort of impractical at times (off-road stuff).
As far as the power rating goes, if you don't plan on running that much power there's no benefit to it at all. That power rating is very 'variable', depends on a lot of things and usually isn't going to be more than a 'ball-park'ish thingy. Don't count on it.
So, would this idea be of benefit to you? Beats me, would it?
- 'Doc
Part of that 'directional' thingy is how far apart you can mount those two antennas. If it isn't at least a 1/4 wave (maybe 9 feet?) then it won't be -as- directional as you might think. Lots of "but's" in it, huh?