Do you mean actually modify the stat by removing the one plug fixture and adding a doubt one? You COULD do this but I just plug a 4 way adapter into the stat which means you can use the stat for 2 mats without changing the fixtures.
That way you havent permanently altered your stat or voided the warranty (in case something on it breaks!)
I have four heat strips in my nursery, all running from one probe, and i dont get any difference
Just note as pendlehog said, if the setups are the same, otherwise one my reach temp no probs, but the other may struggle, or get to hot, so you will need to check both set ups with a thermometer and maybe adjust the stat or probe location.