You can assign multiple snippets scopes to a single filetype. (I've found that altering the filetype tends to break some syntax highlighting).
You can check that the filetype for erb files is indeed 'eruby' with:
:set filetype?
If you're using the maintained fork of snipmate, it looks like you'll want both the eruby.snippets and eruby-rails.snippets from the snipmate-snippets repository (owned by honza, but I don't have enough reputation to link to it here) (see the INSTALL section of the snipmate README for proper setup).
If you are using the maintained fork, I believe setting g:snipMate.scope_aliases in your .vimrc with the following will work for your example:
let g:snipMate = {}
let g:snipMate.scope_aliases = {}
let g:snipMate.scope_aliases['eruby'] = 'eruby,eruby-rails'
I've added a pull request to snipmate to have their documentation updated.