JL audio’s sub inputs are summed. So if you have two of them (which leaves an open input on each sub) you can use them to crossover to the mains AND run the LEF of your processor for movies.
You just setup with the crossover like normal for two channel (between the pre and amp) then run the lef out of the reciever/processor to the open input. This method has worked fine for me for years.
you can then adjust the bass gain on the processors like normal for movies (say plus 5db) and not effect your two channel bass setup. This gives the mains all the power of the subs and also plays the subs louder for explosions etc.