Lot of good combos, CMT's Crossroad series had a lot of good ones. ZZ Top and Brooks and Dunn were great together. Hank Jr. and Kid Rock fit like a glove. Years ago there was the mashup between Aerosmith and Run DMC that marked the end of DMCs popularity and put Aerosmith back on top.
The Zac Brown Band does a lot of crossover stuff with other artists.