Grafting/buddy cherry to another cherry above rootstock
I picked up 2 sweet cherry trees (bing and black tartarian) from a local nursery. They are each grafted onto a rootstock. I'd eventually like to combine the trees together by grafting/budding the bing to the tartarian and vise-versa. Mostly I want to have this "multi-grafted" tree so that it's self pollinating (in case one dies for some reason).
The problem I see is that the rootstocks on these trees are only a couple inches out of the soil before the graft starts, so there is no room to graft/bud directly to the rootstock. Can I graft/bud onto the tree above the rootstock graft, or do all branches need to come directly from the rootstock to be successful?