Cross grafting is certainly reasonable. It's probably a little late to chip bud but you might try that now. If no go there collect scion wood in late winter to try whip&tongue, cleft, bark, or splice graft in April. If that fails try T budding in June.

Grafting onto the trunk isn't a doable plan because you can't cut back enough to force the new portion to grow. You'd do better on a major branch. I guess what I'm saying is be prepared to cut off everything above your graft or bud in order to force the new variety to growth.

You can probably find someone to trade scions but cross grafting will work just as well. I don't really see that much difference in maturity in mine to get me excited about adding more cultivars to extend harvest.

What varieties do you have now?

Thanks fruitnut.

Tree one is a Stella, purchased in 2010. It bloomed in 2012, but frost killed them. in 2013 it yielded close to a gallon of cherries. (I built a simple pvc frame and threw a bird net over it. I put a sheet of plastic over that to keep the rain from causing the berries to crack.) In 2014, the buds never opened due to our severe winter. Same with most fruits across my region.

Tree two is a black pearl planted this spring.

Any other ideas?

Thanks.

Sorry, should have added that the Stella is self fertile. It's pollination of the black pearl that would be a problem if the Stella dies. I didn't go in to that detail in the original question for simplisity's sake.