I think what Glenn meant by "I need two more" was he needs back his two old friends that just died, not two graves, because Oscar had a pick-axe and Axel had a shovel so they were most likely digging the same grave.
I feel like he was saying "I need two more" as in he needs two more new friends... To help him finish digging the grave... Kind of like a "welcome to the team" type of thing