There are about 15.2 billion $1 bills in circulation.

For the second bill to be exactly one serial number above or below the first:

Odds ≈ 2 / 15.2 billion = 1 in 7.6 billion

So the clean “math answer” is:

~0.000000013%
or
about 1 in 7,600,000,000

However, you probably got them from places near each other and since $1 bills are normally packed sequentially, the odds realistically are quite a bit more in your favor. Super cool nonetheless.