First of all, this has nothing to do with Textmate but with how Ruby handles that file path. So no need to fiddle with TM settings.
You are opening a file relative to the current working directory. If you run Test1.rb from the RubyDevs directory Test2.rb is not present in the current working directory, if you open the file from the RubyLearn directory it is.
To make it work from any directory you need to determine the directory of the Test1.rb file and add the Test2.rb path like this:
file = File.open(File.dirname(__FILE__) + '/Test2.rb')
file.close()
Hope this helps!