while i don't run any belt drive WBs, I can tell you that it's probably your right drive belt just being tighter than the left, not sure if that's adjustable, but i know it's replaceable. You might just need new belts, hopefully someone else will chime in here.
But being someone who has a hydro WB they do not pull to either side, they will run in a straight line pretty much always, never had a problem w/ my 36 Ferris Hydrowalk (single hydro btw). The only time it will pull to the right is when I'm using a steel grass catcher, but that's just because of the extra unbalanced weight.
i run one yesterday it was not mine a guy wanted me to see if i thought it was pulling to much.. it did the same thing but he almost had it adjusted out it went pretty straight for a belt drive so he says you can adjust the belts on each side so i would try that sorry i can't tell you how i will try to call him and look at his to give you a little more info i run the hydro scag i only use for hills and with back yard with gates ...
check the tire pressure first. make sure both tires have the same pressure. the manual says to go to 12psi. I think some guys run a little less, but I got mine right to 12 on the dot on both drive tires.
Once the tires are good, make sure there's no chit clogged up in your drive pulleys. the belts have two grooves in them that ride on the pulleys. when I bought my WB, there was tons of crap and dried up grass clogging up the pulleys. this was slippery enough to make the belts slip to the point where it would hardly drive forward. I jacked the back end up off the ground and with the engine running, put it in 1st gear. then i used a sharp screwdriver to clear the crap outta the pulley while it spun. if you do this be real careful it doesnt fall off the jack and drive through the wall of your shop.. I aimed mine toward the garage door just in case.....
next, check the adjustment on the clutch handles. This takes some dinking around to get it right. I have mine adjusted so it takes very little "squeeze" to disengage the clutches. The problem with this is that I expect to have some slippage when it's not wanted. so play around with each side and make sure each side disengages at the same point on the "pull" of the clutch handle. figure out what works best and what's comfy for you to use. To do this, you pop the cotter pin our of the rod that attaches to the handles and it goes all the way down to the belt area. Once the pin is out, you can pop the rod out of the handle then spin it in whatever direction you need to to get your adjustment right. just make sure they're the same on each side. Hopefully that made sense.
Belt Drive's also have a brake at each wheel. when you squeeze the handle, the first thing that happens is pressure is let off of the belt allowing it slip and this take power off of the drive wheel. You probably knew this already. then, when you squeeze harder, the brake will grab that one wheel which makes the thing turn real fast since it all but locks up that one wheel. you want the brakes to be tight enough so that that they grab and keep the thing from moving when the handles are in their locked positions. but you need a little play between the point where the belt disengages and the brake takes over. if it happens at the same time, it'll make turning kinda tough. if you have a little slop in between those 2 points, you can sort of yank back on the one handlebar to help turn and keep from tearing the turf up. So if you want to turn right, you'd lightly squeeze the right handle, which will disengage the clutch and take power off the right wheel. but you don't want to squeeze so hard that you apply the brake on the right wheel, if that right wheel can "freewheel" while making the turn, you can let the left wheel bring the machine around while you pull back on the right handle bar a little. again, i hope that made sense.
ifs not hard to adjust these things, it just some fuddling around to get it right. im sure there's a "right" way to do this, but I just used the old trial and error method. be sure to give everything a good cleaning and grease job while youre at it.