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 is a really cool idea. So it should automatically make its own apples? (Assuming there's enough wind and pollinators?)
Yes it should. I picked varieties that bloom around the same time, so they should pollinate one another. It's like having 3 separate trees, they're all just connected to the same root stock.