Is it normal for twin motors to sound slightly out of sync on the bench?

Flown a few twins and that low throttle whine difference is really common. Two ESCs never end up perfectly identical even after calibration, and brushless motors have slightly diffrent timing and bearing tolerances, so at low rpm you hear it. As throttle goes up the difference becomes a smaller percentage and they blend, which matches what you're hearing. On most setups it's nothing to worry about.

The thing that actually matters is static thrust mismatch, and the whine isn't a good indicator of that. What I do on the bench is run both motors up to about half throttle against a scale or just feel the exhaust blast with my hands at the same distance behind each prop. They should feel pretty close. If one is noticeably stronger, swap the props between motors first, since even two props sold as the same thing can differ more than the motors do. Also check both props are the same rotation direction unless you specifically set up counter rotation. A lot of guys accidentally end up with both spinning the same way and never notice until the plane pulls hard one way.

In the air, small thrust differences mostly wash out once you're moving. You trim it with a click or two of rudder or aileron and forget about it. Where it bites you is exactly the moment you're worried about, the launch or rollout, because there's no airflow over the rudder yet to fight the yaw. So do a full throttle runup while holding the tail or with a helper, and make sure it tracks straight. If it pulls, note which way and be ready with rudder on the launch. One more thing since you're hand launching a 10 pound twin, make sure both ESCs are arming together every time. With a Y harness sometimes one arms a beat later, and if that happens right as you throw it you'll get a lurch. Cycle the throttle a few times on the bench and listen for them both beeping ready before you commit.

This is helpful because I'm slowly putting together a twin myself and hadn't even thought about two motors needing to match. One thing I got caught on early with my first electric plane was a prop on backwards, it spun fine but pushed barely any air. So now I always spin each one by hand and watch which way the blades bite. Quick question though, do the two props on a twin need to spin opposite directions on purpose, or is it fine if both go the same way if the plane flies straight?

Mine did the same thing on a twin I built a couple summers back. Drove me nuts til I plugged them into my programming card and found one had a different timing setting than the other. If your ESCs are programmable, it's worth checking they match on everything, not just the throttle calibration. Fixed the whine right up on mine.

Back in the glow days we matched props by balancing them on a knife edge and picking the two that sat closest to level. Same idea works here. I also ran each motor solo through a watt meter once and picked the ESC that read closest to the other, rather than trusting the Y harness to sort it out. Another thing from expirience, check your motor mounts. I had a twin years back where one mount was sitting a hair loose and the motor would flex under load, which changed the thrust angle just enough to feel like a motor mismatch. Took me a week to find that one.

Kind of a newbie here so take this with a grain of salt, but I ran into something similar sounding on my foamy. Turned out my two packs weren't matching, one was a bit more discharged than the other, and the ESCs were doing funny things at low stick until I put a fresh charge on both. If you're running them off seperate batteries during bench tests that could be part of what you're hearing. Might be nothing, just something dumb that bit me once.