If that's the case, it will never happen, since you'd need another differentiation like drums, pro drums, pro drums double bass or something, which would be like adding EX+... Which HMX has said will never happen.
Also, it'd be stupid to pay a dollar for a handful of bass notes, as opposed to actual tabs for two instruments.
^ So just get the 2x Bass version, and encourage your friends to do the same. Almost no randoms have RBN songs (or is this different on Xbox?) so I don't think you'd miss out on much in terms of online compatibility.
I feel your pain about the HDD space, d-carless, since I usually buy both versions if its a song I want the double version of, but I upgraded to a 250GB when my old xbox killed itself..