Can I get away with 12" round supply and return for 2 ton?
Yes, my “Ductulator” says a 12” diameter duct (with insulation on the exterior of the duct) is a little small, but barely adequate for a 2 ton unit.
If I was sizing it I’d use a 14” diameter with a friction loss of .09 per 100 feet of duct on the SUPPLY side and .07 on the RETURN side.
However, your 12” duct will make the air travel a little faster than recommended, but it will not “whistle”.