My Theremini doesn’t work right when it’s near another Theremini, Etherwave Theremin or any other Theremin.

Theremins are sensitive instruments that operate via proximity sensing antennae. When using more than one Theremin, you should maintain a distance of 10 feet (3 meters) between instruments. 

Also, please be aware that if a metal object is placed between your Theremini and another Theremin, the metal object can cause coupling between the two and cause poor operation.