You can add a send from Bus 1 to Bus 2, but if you send Bus 2 to Bus 1, you are creating a feedback loop since the audio in Bus 2 includes Bus 1 audio.
What if you created Bus 3 and send each of the buses there instead? Would that accomplish what you are after?
You may need to make them pre-fader sends and set their Volume to zero to keep the effect "pure"