the wont perform as well, as both speakers will be trying to play full range, and as it can only play certain ranges well, it will make them worse SQ and prob SPL. also you will have unneeded overlap of freq.
Also they have nothing to do with a sub! - (i dont see the connection looks as if you are confused)
Also crossovers shouldnt be too hard to install, just use velcro or somthing similar to attach them in the tops of the foot wells.
Rorrie