The DC for jumping 20 feet is an Acrobatics check of 20. But what is the modifier if you are standing on a 1 foot beam and jumping to another 1 foot beam?
First, if the beams are oriented side by side and not a long beam with a 20' gap in the middle the DC is actually 40.
For a running jump, the result of your Acrobatics check indicates the distance traveled in the jump (and if the check fails, the distance at which you actually land and fall prone). Halve this result for a standing long jump to determine where you land.
Thus, DC20 only gets you 10feet, you would need a DC40 to get 20feet if the beams are oriented side by side.
As for a modifier for starting jumping off of a onto a different 1' beam, there isn't one in the rules. If it was less width than 1' or the beam was slippery then a separate acrobatics check would be required.
- Gauss