Alan, in order to do this correctly, you need to match amplitude levels, bring acoustic centres pretty close and, of course, choose the optimum xover frequency and, not least, the best acoustic xover slope... a few other things perhaps, but the above are really necessary.
So, can you find / buy an inexpensive tool ($¬200) in the form of a Behringer deq or their dsp? OK, it's audiophile non-approved BUT you can use it to determine all the optimal parametres of yr system. Once you know that, go ahead & buy the best xover. At least you'll know what to do with it!