The only interpretation I can think of that fits is the one Peter suggested: the first brother knows that the second brother wants to copy whatever he does, so he jumps off a bridge in order to hurt or kill the second brother, who will also jump off the bridge. This scenario echoes the idiomatic expression, "If all your friends jumped off a bridge, would you do it too?" (See J.R.'s comment on the original question.)
Also, if the second brother bursts into tears every time he and the first brother aren't doing exactly the same thing, I can see why the first brother would want to get rid of him.
That being said, I don't think it's a very good joke, for a few reasons:
- The heavy emphasis on "TWO days" in the punchline is pointless and distracting. It has nothing to do with the rest of the joke, aside from the fact that there are two brothers.
- The second brother is not established as being active enough to actually copy his brother in jumping off a bridge. Sure, crying because the first brother ordered a beer is weird and over-emotional, but it's also passive. If he's really determined to copy his brother, why doesn't he just order his own beer? What's stopping him?
- If the first brother survived jumping off the bridge, wouldn't the second brother survive as well? How could the first brother know that the jump would kill his brother but not him? On the other hand, if he was only trying to kill himself, why doesn't the punchline tell us whether the second brother copied him or not?
I tried re-writing the joke. It doesn't quite follow the formula of the original joke, but I like it.
There were two twin brothers sitting in a bar.
One of them asked the bartender for a beer and the other one began crying.
"Why are you crying?", asked the first brother.
"Because we're twins", answered the second brother. "So we should both be drinking beer."
So the second brother bought himself a beer.
After a while, the first brother asked the bartender for a whiskey and the second brother began crying again.
"Now why are you crying?", asked the first brother.
"Because we're twins", answered the second brother. "So we should both be drinking whiskey."
So the second brother bought himself a whiskey.
After another while, the first brother asked the bartender for a tequila and the second brother began crying AGAIN.
"NOW why are you crying?", shouted the first brother.
"Because we're twins", answered the second brother. "So we should both be drinking tequila."
So the second brother bought himself a tequila.
Later that night, the first brother jumped off a bridge. He died when the second brother landed on top of him.