Well, I guess that's for them to decide, ultimately.  The textbook definition for miners is probably something along the lines of "it appears we can make money from this", heh.  

Obviously from their standpoint, if the fork falls flat on its face and no one uses it, then it's not going to be worth their consideration, let alone time or effort to mine it.  Conversely, if there's a thriving and active chain with steady diff, they'll probably do some calculations and see how financially viable it looks for them.  Or, if the situation is somewhere in the middle of those two extremes and it all looks a little uncertain, dividing your hashpower will probably look like a less attractive option than just sticking with the security of a "known quantity" and focusing on SegWit2x alone, which is probably a big enough job on its own to keep them busy.  But again, I'm sure they'll do their calculations either way to see what's best for them.