Tune Up is supposed to be a "Self, Allies" support skill, with a range of 10m. However, I often can only Tune Up the ganger using it. I've been able to Tune Up another heavy (not the one using it) with it, at times, but it seems kinda random.
Playing house Orlock, on PC, and the behavior is verified mission after mission... (playing operations, but I highly doubt it has any relevance)