I like this concept, the leads were sturdy, and the setting suited 48Hour filmmaking very well. It allowed for a very clean look and neat camerawork/editing tricks that effectively conveyed coffin life and the depth of the grave.
However, a conversation-focused short relies on sharp dialogue, and that's ultimately what's missing here.
There were numerous small lines that could have been boosted to show the older character's loneliness and/or time he's spent in the ground. Comments like "I think I'm next to you" for example could have instead played off how well he got to know the graveyard ("You're in allotment B5, segment 12 which puts me 45 degrees on your left"). The film also establishes that he died in 2013, which means he wouldn't know what the hell a Brexit was. That felt like an opportunity gone missing. (I did like the Batman Avengers line, though.) Also, when you have a subtitle that says 'Two Months Later', you don't need someone restating "We've been stuck here for two months now..." - just cut straight to "...you still haven't told me how you died."
That last point, in particular, could have been expanded on earlier with the character trying to bring the topic up but having the other guy avoid it. That would create a tease to make the audience wonder why he's holding that info back throughout the film. As it ends up though, it goes straight from 0 to 100 in a single instant, and is less effective as a result. It's also a pity that he mentions how he's had no one to talk to for years, and yet his first calm-and-collected interaction doesn't really reflect this.
I liked the ending though. It seems inevitable that these two characters would go into a verbal flurry about death, revival, and how much it sucks to be in their situation with no explanation. But, hey, they have each other, and that soothes the pain. I found that bittersweet. I just wish that when they went out to scare that person, we actually got to see that person being scared.