[Suicide Junkie]

Repairs are sector-wide.
A single ship can repair any number of other ships simultaneously, and multiple repair ships can repair a single ship.
They need only be in the same sector, not fleeted.

Repair rules can be set in the empire options window (under the repair priorities button). This lets you decide which components are most important to repair.
If there are no specified rules that decide between two damaged ships, it goes by the order in which the ships were built.