I am currently happen to be experimenting some sort of double rootstock grafting on avocado, primarily encouraged by what the benefits that I read elsewhere and here; also Simon's successes have been encouraging to me.
I did try the same method Simon used, once on mango, but graft didn't take and died for reason unknown to me. (I think it's because of my sloppy wrap.)
So, a couple weeks ago, I tried a little different method on avocado, (running out of mango rootstocks) I spliced both sides on the scion to meet 2 slices of the 2 rootstocks.
Don't know if anyone ever tried this method, but I thought doing this way will give me a better change of the graft to take (double sided contacts), but wrapping those 3 branches together is not as easy as doing a regular cleft graft...I found out!
So, I am still waiting the result of this, I'll let you know the outcome in a few...(weeks?)...knock on wood!