Well it could help spread the heat a little, but if your getting the thermostat soon anyway, I would say just keep it on the side you intend on having it on later, so your bp won't be disturbed by the change when you get the thermostat. Also don't forget to NOT stick the UTH right on, its not fun getting them off Ive tried lol. If you try to pry them off they usually don't work after. Just tape the sticky side with aluminum tape. 78 on the cool side won't hurt him for a week or so, but for a long period would not be good. I have my hot side on a ranco thermostat and my cool side on a lamp dimmer. At low temps, the dimmer fluctuations aren't as bad.

And BTW it sounds like your doing great so far! Good for you to get everything together ahead of bringing your bp home!