Depends, but you are getting into the arcane knowledge of speaker building. You may want to ask this question in DIYaudio's multi-way forum.
Generally, if there is enough overlap in the range of the two, then yes, but no idea what you have for crossover in the bottoms or uppers, and how much you'd have to modify.
Your easiest path would be to remove any high-pass filtering in the bottoms and use an active crossover. If that's too confusing, you definitely want to head over to the DIYaudio group. :)
