There should be no problem doing that. It's the same thing as using a "Y".
As luck would have it my "Y" connector gave out this weekend - Patchecd the two channels together as discussed and the worked like a charm - Sound was great and still had the option of second channel out... Which got me to thinking.... since my JBL JX125's are rated at a low end of 35hz, Rather than running it through the x0over first, might I take the second output - run it to the x-over, dial it in and then run to my subs (they are only rated for a bottom end of 42hz. Might give me a little lower thump? - Just trying to think "out of the box - Rod
I would keep the xover ahead of the amplifier. Don't try to loop back from the amp to the xover just for the subs. The Top boxes would be trying to do everything including all the sub frequencies. Since the JRX boxes start rolling off pretty good below 100Hz you wouldn't gain a thing.
Keeping the xover before the top, and setting it for 100 Hz, will give you the best sound you can get with your boxes. In this way the top box will be getting the frequencies it is best suited to be reproducing and the sub will be getting the right frequencies as well.
David - Thanks for the info - You da man! - Rod