Question

What I would like would be something like this:

gem 'rack', '1.3.3', '1.2.4'

So that when gems require different versions of rack, they are all appeased. Is this possible?

Answer 1

You can set an intervall of allowed gems

gem 'rack', '<1.3.3', '>1.2.4'

It will load the most actual one inside the selected intervall.

But I don’t think you can require different gem versions.
If a gem would be loaded in different versions, each class and module must get it own namespace to avoid to overwrite the methods of the gem.