Water powered Back up sump discharge
The water powered sump pump, for all intents and purposes, can be considered a second sump pump in parallel with the first, and can be plumbed that way.

They can share a discharge pipe as long as the pipe is rated to carry the load of both pumps running simultaneously.  Even if they should never, in practice, run at the same time, it would be inappropriate to attach both to a discharge pipe that could only handle one at a time.  Most pumps I've run across seem to be installed with larger piping than they absolutely need, though, so you're probably ok combining them.  Be sure to use two check valves, one for each pump, before they combine into the one discharge pipe.