Hi Bob,
The topic " Who are you going to call if you need 10 thousand grafts completed? " links to over 40 videos by Ken Coates specific to grafting cherries. Ken addresses all of those questions in various places.
Based on his videos and my own experience with plum topworking:
Cherry scion grafting should be done very early in the growing season. This precludes bark grafting since you would have to wait until later in the season for the bark to slip. Bark grafting is fine for apples, not so good for cherries. For topworking, Ken demonstrates what he calls a wedge graft and a side graft depending on the diameter of the understock. He makes the wedge graft look easy but it takes a lot of practice to cut the scion and the slot to the correct angles. If you can execute it, it works very well. I have had success topworking plum trees with his method.
Grafting low on the tree while a large amount of growth remains above the graft will result in very slow growing scions due to apical dominance. So grafting just one side of the tree is not recommended.
Trees over ten years old have higher odds of just outright dying when you attempt to topwork them.
Since your tree is worthless in it’s current condition I suppose you have nothing to lose. I would cut and graft both of the trunks and leave some of the lower growth (nurse limbs) until the scions grow out. The nurse limbs will help to keep sap from flooding your grafts.
If the scions don’t take but the tree still lives, it will almost certainly send out water sprouts that are prime grafting locations for next year!