It can be a very tedious process, but after adjusting the tension, try lowering the bike to the floor before you tighten the axle nut to final torque. This will generally prevent the shift you are getting by allowing the geometry of the swingarm and the weight of the bike to keep both tensioners loaded. If you apply final torque with the wheel in the air, you are putting in enough twist to move one side slightly off the tensioner which is why you then see the belt move. If you got the tension set right, you're done. If you still need to adjust it, then it's back up & down on the jack some more!
Good advice. And it helps if when do the final torque, torque each side against their adjusters. IE; one wrench up and one wrench down in the proper direction. I think 90#'s is plenty of torque for the final adjustment. Ask Joe on Squared Away, he's done a freight car load of belt adjustments.