Yes, you need 2. You can have 2 with probes or 1 with probe and the other without. I have the one with probe over the glass right above the uth. On the cool side I have one without a probe, it just seats on top of the substrate and it measures ambient temperature. I found out with a temp gun that the cool side always matches the ambient temp. I only worry about the temperature on top of the uth, because you can burn your snake. Mine reads 90f with the thermometer probe and 92 with the temp gun (that's right on the glass, not the aspen I use) If I read the temp on top of my aspen with the temp gun it read 86f... and I'm not worried about this temp, I know underneath this is 92f. My snake can burrow to reach warmer temps if she wants to.
But Kevin, you don't want the glass at the bottom reading 108f, just so your temp on top of your forest floor read 90f...this is a mistake. Your snake will burrow and touch the glass that's 108f and burn itself.