In article , Kai Glaesner
wrote:

What do you need? An additional portable 2-place intercom? A Y-cable
extension?

Plugging a second intercom into an existing intercom will result in
feedback if the mic plugs are plugged in. This is especially true if
you have the second intercom plugged into the pilots side mic jack. You
may not hear it within the airplane, but if you transmit, all anyone
will hear is a squeal.
Two Y-cords may work, or not , depending on the impedances of the
headsets.