Here Paul is speaking of Epaphroditus as if he were a soldier. He means that Epaphroditus is trained and is dedicated to serving God, no matter how great the hardship he must suffer. AT: "fellow believer who works and struggles along with us" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The cause of the sorrow can be made explicit. AT: "the sorrow of losing him added to the sorrow I already have from being in prison" (See: [[rc://en/ta/man/translate/figs-explicit]])