Do I need a trim tab system on my twin tail?
Yes a trim system would be a most excellent idea. If you can trim the aircraft such that it just flies straight on when you release your hands and feet off of the flying controls, the flight will be more enjoyable than when you need to keep pulling one way for the duration of the flight, like walking while holding up a briefcase in front of you.

Trim adjusts the neutral point of the flying controls: the point where the stick returns to when released.