What is the difference between a R6 & R6.2?

The obvious one is that the R6.2 has had its max shutter speed increased to 1/2000th - a stop faster than the R6's 1/1000th.

There are a few other subtle differences as well. According to Bill Larsen:

… The exposure counter was relocated so it is adjacent to the metering selector switch and is magnified. The shutter blade mechanism was changed is lighter in weight and supposedly has better accuracy. They both have TTL using the same Metz adapter. Both are basically mechanical cameras and will function without a battery. For flash use, both have a electronic shutter release that is activated when the shutter speed is set on "x". Both require a circular polarizer since metering is through the mirror.
I suspect that one of the reasons the R6.2 was about the same price as the R8, is they basically had to build a camera with a dual electronic and mechanical system with the mechanical system capable of operating without batteries or in the case of battery failure. I only have the R6, so this is all to the best of my knowledge and research.

BTW, the R6.2 was discontinued at the end of Jan 2002. This has resulted in prices for mint-condition used bodies increasing to the point where they are @ $US 1600 each. Ouch.