Or will the combiner eliminate that problem?
The crosstalk from one channel to the opposite channel is the drawback with the simple passive combiner. That's the reason I cautioned Huck away from that type.
Huck has one channel of his BFD used already for a feed from a mono LFE signal on system #1. He wants to feed the other BFD channel from system #2 that doesn't have a mono LFE signal. So, he must combine that stereo mains signal first before he can accomplish this.
He will of course have to get creative with the BFD since that combined signal will be full range. Presumably he has defeated the LPF crossover in his sub as a consequence of feeding LFE from the system #1, and so a crossover in addition to equalization will be required from the BFD filters for system #2's full range signal. Certainly possible.
Personally I would have accomplished the combining and the LPF crossing with a device like the Paradigmn X-30
, since that would have solved both problems at once, but they aren't cheap. I think they're around $150, but I've seen them on ebay for less.....
Combiners are a juggling act with impedance matching. A passive combiner using simple resistors will either have an input impedance that is too low, causing crosstalk to the opposite channel as you suggested, or (if you modify it to have sufficient input impedance) it will have too high output impedance causing distortion. The solution is to impedance match with active components or a transformer. Fairly simple devices.
Hopefully the cheap device I suggested will work fine. They never supply enough information, but you can't lose for $20.........