Wiring new lights in garage Question
You have a 240V ( line to line) short with these two breakers. Somewhere down the line you have a homerun ( wire straight from the panel) that is connected or touching another phase from your panel. If you look at the breakers, one probably has a black wire, and the other probably has a red. ( hoping this is not a really old house with old wire) The best way of figuring this out is to see what is on the breaker that is always on. Then open the box, (outlet or light that is run off the working breaker) that is closest to the panel ( hopefully) and see if you have a cable that has three conductors ( plus ground). If you do, somewhere down the line the black and red are getting connectivity. You will have to trace it throghout the circuit and see where this is happening. Good luck.