F-350,I removed the left head and the shop found 2 cracks, should I go ahead and replace both heads or just the known bad one?

The machine shop said the left head had 2 large cracks probably caused by overheating. Should I just replace this head (it was leaking exaust and fluid and was the reason for taking it to the shop) or is the other head probably cracked too and I should just go ahead and replace both?

My car has 115000 miles.
My car has a manual transmission.