I'm afraid Blooie has been MIA for some weeks. My opinion on your question is that more than likely you can mix the new with the old since the old won't even be 2 weeks. As to the 0°F temps, no idea. They SHOULD be OK if the pad is set at a height all the little peepers can get their backs up against it without stretching.
Thanks for the reassurance @bruceha2000. Didn't realize Blooie was MIA. Glad mixing them shouldn't be an issue either. I am over ducklings and chicks inside my house lol. Honestly though, this MHP method has been amazing. I wish I would have started it with the ducklings as well, but only have the one heating pad. I really need to get another one as a backup though.
I have one of those cozy coop flat panel chicken heaters but the ducks prefer the ceramic heat bulb over that, so I am not sure the MHP would be received any better.
Temps of 1* & 3*F make me a bit nervous, but luckily there is a camera set up in the coop and it has sound so if I hear excessive peeping then I can always move them to a warmer place or add some supplemental heat.