the setup

Two jokes walked into a bar.
The bartender was a riddle.
She said "What'll you have?"
One joke said to the other

"That is quite a riddle isn't it.
What'll you have? One of us
lies and the other tells the truth
so what you order I'll either
do the opposite or

the same

That will be the solution"

Then the bartender was like
"We only serve one drink here. GUESS"

The first joke replied "Knock knock."
"Who's there" replied the bartender.

"A certain number of light bulbs,"
replied the second joke. "Guess"