Do I need a separate dishing tool for building wheels with park tool TS-2.2 truing stand?
A properly aligned TS-2 truing stand will show dish, as well as alignment.  You rarely need additional adjustment after using it.  However, if the stand is out of alignment, you may need to re-dish the wheel.  

The easiest and most accurate check for TS-2 alignment is a properly dished wheel.

The best way to ensure that the wheel is properly dished, and thus an accurate alignment check, is with a dishing tool.  

Bottom line, if you can afford the TS-2, a dishing tool shouldn't be a major expense, and is worth the cost, even if you only use it as a final, double check of the truing stand.