Sometimes something a player wants to do is a combination of two different skills. For example, being able to spot the difference between two very similar types of plant. Or stealthily walk along a tightrope. For these situations, players can roll a combo skill check.
To roll a combo skill check, the player rolls both skill checks individually, and takes the average of the two results.
In this way, if a player does exceptionally well at the Acrobatics check, they don't need to be as Stealthy, since they were so good at Acrobatics. This mechanic can be used in any situation where it feels like two skills are being tested at the same time.