What's an easy way to see if they are canceling each other out? Play a loud bass scene and measure SPL with all subs on, then turn off the lessor subs and see if its higher or lower? Plus see which sounds best?
That’s one way, but there’s unfortunately not an easy way that’s accurate or consistent.
The problem is not only with cancellations, but peaks too, where some frequencies will support each other. Testing something like that with largely unknown(frequency wise) material and even worse, the human ear as a measurement tool, would be guesswork really. You’d have to know exactly all the frequencies in the LFE channel and bass material during the passage. And no doubt it would be different for different films.
To know for sure, you’d want to use rew and a good mic. Take sweeps with subs on/off. Honestly though you’d need to know how each sub performs in every area of the room, or at least the locations in question.
That’s unfortunately a very deep dive that takes a lot of time and knowledge.