What are the viv sizes? I'd put the thermostat probe on the bigger mat, and then watch both temps to make sure one isn't too hot and one isn't too cold. If you could get something like 86F on the big mat and 80F on the small mat, that would probably be acceptable for the snakes. Ideally, the mats and vivs would be the same size. I've found that even with same size vivs and same size mats, if the vivs are stacked, or one is shelved higher than the other, the top viv will have a temp that is 2-4 degrees F higher. If yours are stacked, you can even things out more by putting the viv with the bigger mat on the bottom.