Ill check this when i get home but why do i want one to have a strong flow and the other to not? Just trying to understand the reasoning lolPut your hand under the pipe that’s not causing issues. Is it a strong full siphon?
Then feel the flow under the pipe with an air valve causing noise/ bubbles. Is it also strong flow or is it barely strong enough to feel? If this pipe has strong flow, it shouldn’t and there’s your problem.
This can be fixed by a gate (or a ball) on this channel.
I’d also experiment valving down the return pump (on the output side) and trying to tune it so that you have a full siphon in pipe 1, and just a little extra flow in pipe2( the one with the air valve)
That would also potentially solve the issue.