Why can't I play my 2x Bass Pedal version track with my friend online?

It's likely that your friend own only the regular version of the track and does not have the 2x Bass Pedal version. From the perspective of online play, these are two entirely different songs. You will only be able to play a 2x Bass Pedal song with an online friend who also has the 2x Bass Pedal version.

Please keep in mind that this is ONLY for legacy Rock Band titles. There is no 2x Bass Pedal feature in Rock Band 4 at this time.