I assume that there is a compelling reason to want to go to Cancun in THE hottest part of the summer there, right at the start of hurricane season?
Assuming yes, and also assuming that you are OK with paying to re bank the miles, should another seat not open up, I'd get the seat that's available now.
Is it that there are no seats in front, or no seats? Even if there's one in front and one in back, book two. It's easier to upgrade the lower one than to start from scratch.
If that's the case, worst case, you pay for premium economy for the second seat, and switch off, one flies F on the way there, the other on the way back.