Any problem in using two battery maintainers at once?

Keriminy, don't ever try going to a Fellini flick. Number one, what is the frequency of the unit? It has to be pulse width modulated. My experience with mix and match is shortened lifespan of the components. Sometimes alarmingly shorter. I used musical instruments to simplify what can happen with high frequency devices. Imply the wrong combination of frequencies and you can create harmonics. You don't want to know about that...