So here's the flow I made that should do what you want.
I'll break down what's going on below:
1a.) WHEN Date & Time is 5am (repeat every hour, end repeat never).
1b.) ONLY IF First Temperature Thing's temperature is greater than -40. This -40 doesn't matter, just make sure it's always going to be true. Put your Master Bedroom Relay here.
1c.) ONLY IF Second Temperature Thing's temperature is greater than -40. Put your Living Room Nest thing here.
2.) THEN Math Perform math operation on two values, first being the first temp, second being the second temp. We just want to subtract them.
3a.) THEN Math Continue flow based on comparing numbers. Compare the "function output > 2".
3b.) THEN Math Continue flow based on comparing numbers. Compare the "function output < 2".
4.) THEN turn on your fan. I used notification here because I don't have a fan to test with.
So one thing to note here. Look at the Date & Time thing. It's going to repeat every hour. So that means it'll only be checking the temperatures and triggering the flow once per hour. If you want to do it MORE often, do the below flow.
If you want to check your fans every 15 minutes instead of every hour, here's how you'll do it. You'll need two flows. The first involves Date and Time and Variables.
The second flow will be the same as the one above, only instead of using Date and Time you'll use Variables.