SYSTEM REFERENCE DOCUMENT 3.5
When more than one character tries the same skill at the same time and for the same purpose, their efforts may overlap.
Often, several characters attempt some action and each succeeds or fails independently. The result of one character’s Climb check does not influence the results of other characters Climb check.
You can help another character achieve success on his or her skill check by making the same kind of skill check in a cooperative effort. If you roll a 10 or higher on your check, the character you are helping gets a +2 bonus to his or her check, as per the rule for favorable conditions. (You can’t take 10 on a skill check to aid another.) In many cases, a character’s help won’t be beneficial, or only a limited number of characters can help at once.
In cases where the skill restricts who can achieve certain results you can’t aid another to grant a bonus to a task that your character couldn’t achieve alone.
It’s possible for a character to have two skills that work well together. In general, having 5 or more ranks in one skill gives the character a +2 bonus on skill checks with each of its synergistic skills, as noted in the skill description. In some cases, this bonus applies only to specific uses of the skill in question, and not to all checks. Some skills provide benefits on other checks made by a character, such as those checks required to use certain class features.