I've never grafted.
1) Does this seem like a reasonable idea?

The younger tree has very few branches and no lower branches. Because this tree will remain small, It would be better if it had lower branches.
2) Can I graft directly onto the trunk?

3) What type of graft would you suggest?

4) When is the best time to collect scion?

I might be better off buying a third or even fourth cherry variety to graft on to increase my odds of pollination while both trees are still alive.
5) where is a good place to find high quality scions, preferably with a wide choice of varieties?

6) Any other thoughts?

Thanks.

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?

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.