Is it possible to create a fieldtype that creates a many to many relationship between two entry sections?
There's still a few kinks to work out, but I developed a plugin that solves this problem using Craft's native Many to Many relationship that already exists.

Basically, I've created a field type that links the relationship from the other side of things and allows you to add entries to it.

You can find it here:
https://github.com/page-8/craft-manytomany

Thanks @Brandon Kelly for the help and guidance.