Do you know how much they try to push their own changes back upstream?

Yes, Aman and Koichi work very closely, the main point of difference at the moment is the method cache patches, Koichi is working on getting something similar implemented in MRI

Why something similar but not the same?

That is a very good question, and I also I'm curious to find out

As if the Ruby Community has more then enough resources to improve on TWO Compiler / Interpreter at the same time. Isn't it better to work together then to reinvent the wheel?

Speaking of compiler, the JIT for Ruby development has been very quiet for months.

Aman Gupta (who wrote this PR) is a Ruby core committer.