Positioning counts for a lot. If one is closer to the 'inside' of the room and the other is nearer a door, window, or vent, then that one will be cooler. Also, if there are any outside walls in the mix, that affects things as well.

I have a rack and a separate tub (on a stand) next to it. The stand is closer to the room door and the large window, and the rack is in the warmest corner of the room. Naturally, the solo tub is about 2-3 degrees cooler than the rack temps, which also vary from bottom to top.
Soon, I'll have another identical rack beside the first and all 8 enclosures will be on one Herpstat (6 tubs 3 in each rack and 2 tanks one on top of each rack). Hopefully the differences won't be too big, but you have to expect variations of some sort.