I don't think that it's possible to do that because each engine is its own container and you can't reach across between engines to do what you're attempting to do.
Instead, define a module which you can include in both contexts which define the same concern:
module CommentableConcern
def self.included(base)
base.instance_eval do
concern :commentable do
resources :comments
end
end
end
end
I think this is the only way you can accomplish that.