Milwaukee electrics (at least the Little Joes) were able to MU with diesels, although it appears the electrics were always in the lead.

A couple years ago, I saw a passenger train in the main station in Hamburg, Germany with a diesel leading and an electric behind it, with the pantagraph up. I don't know the situation, but I assume the train was going to run on non-electrified trackage at some point in its trip and this was considered more efficient than an engine change down the line.