Interference?

I am looking to purchase a simple mixer and keyboard amp but am concerned that if I connect both my theremins into the same mixer they will interfere with each other.

I noticed that when I connect my theremin to a powered speaker, it grounds itself through that connection (as the power adapters only have 2 pins, no ground), making using my loop pedal a problem as the stomp thing lets the theremin ground through me and that borks the pitch control. 

I know little about the tech and am trying to put together a flexible system that lets me add other things to the audio stream and manage it, but not set up a whole multi-track recording studio. Advice would be welcomed.

As far as I know, you ought to be okay with connecting your theremins to the mixer UNLESS you have them both powered on at the same time. Any time you have two or more theremins turned on in the same space - even if theyr'e connected to separate amps and plugged into different outlets - they will interfere with one another like crazy.