Best way to implement different bootstrap versions on the same page?

You can do'it realy easy by isolating Bootstrap with LESS, there is a good tuto to do this by "Chris Youderian". https://formden.com/blog/isolate-bootstrap

Depends on how hacky you want to get. If it's just one feature, just copy the part of the .css from bootstrap 3.3 you want in its own css file and import this css file after the 2.2 version is loaded either in your build script or after your 2.2 version in your tag.

If you have a more specific question about which feature (if you're including js files as well) then we'll have to dig deeper.

Warning: You have to be careful with what you're overwriting from the 2.2 as it can break other css parts. And as @DavidG pointed out, I would avoid doing this entirely.