You would be better off rewiring the system to be honest sound quality would be better.
Are you going to use componant speakers up front ?
If you are they will have their own crossovers you can fit behind the footwell speakers you would just need to rerun wires into the doors or trace whats there.
Crossovers will remove at frequency so if one cross over all ready removes the frequency the other will not need to but it shouldn't cause issue i wouldn't have thought.
Is the H.K sub box the single 6.5" speaker or the twin 5.25" ?