Our apple tree wasn't doing much of anything (it barely even ever flowered), so I finally cut it down and bark grafted 3 different varieties onto it - Liberty, Ashmead, and Cox Orange Pippin. These should all be able to pollinate one another. We also have a Honeycrisp tree nearby that should benefit from this, too.
That's pretty much it!
Check out the whip and tongue graft. Very similar idea to what you described but it's much more secure and I want to say more of the cambium layer is joined together.
How hard is that to cut with a normal knife?
Not difficult if it's sharp. I use a grafting knife, but a pocket knife works just as well.
Bark grafting may pose a problem since you'd be cutting into the bark of the trunk, but it's easier to do in the spring.
You may be other tools (like a saw) in order to do certain types of grafts.
You can get the angles to match up pretty well by hand? I guess you could probably practice on random twigs.
Yep. You just kind of eyeball it. I start with the branch I want to graft onto first, then cut the scion and compare it to the branch and just make adjustments as needed.