Same could be said of the advice in this thread.
Wait for your partner. You aren't doing anything unethical/illegal by waiting and it's a legit explanation should someone ask you.
Waiting (like in the bathroom) and being inline together should not be a problem.
Trying to process together could be. The OP did not mention if it was from the same destination or not. As I and others noted, why risk some overly bored CBP officer writing a novel in the system about the travelers, affecting future flights possibly, when the OP can just wait on the other side.