Example of play: Kyra the cleric is making a Diplomacy check, and Valeros the fighter declares he will aid Kyra. Kyra's result is a 10 (natural 2 + an +8 bonus). Valeros' result is instead a 18 (natural 19 + a -1 penalty). The end result, a 12, is a failure. Bad feels are had all around; Kyra's player, although she's the one who started the Diplomacy, feels overshadowed by Valeros' result. Valeros' player thinks, why didn't I just make a Diplomacy check on my own? I could have succeeded if Kyra aided me instead!
I'm thinking of adding this house rule to my game. I'm aware it messes with some things (halflings' helpful trait, for example), and while they haven't shown up in the player characters at my table, I'd like any ideas you have on addressing them.
Aid Another
You can help someone achieve success on a skill check by making the same kind of skill check in a cooperative effort. Roll 1d20. The character you're helping can choose to use your roll instead of their own, then apply their own modifiers to the check. The character must decide to do so before the GM reveals the results of the check.
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, such as trying to open a lock using Disable Device, you can't aid another to grant a bonus to a task that your character couldn't achieve alone. The GM might impose further restrictions to aiding another on a case-by-case basis as well.
I think this is a fix in search of a problem. The scenario you presented is actually something that could happen in real life. Sometimes the person helping may see a better way to do the job, but the primary character does not take advantage of that. In some cases the secondary character may screw up the skill if he tries to force his way. Let’s look at a different skill as an example. A character is attempting to disarm a bomb (trap) and goes to cut a wire. The person assisting him disagrees with what wire should be cut. The secondary character tries to grab the clippers and cut the wire he thinks, but in doing so knocks the first characters hand causing the bomb to explode.
It would also completely alter how having multiple characters helping works. With the existing rules it is possible to achieve a higher result through Aid Another than can be done by any individual. For example if I have 10 people using Aid Another on a skill that can give me a +20 to the roll in addition to my own bonus. Normally you would not have 10 people aiding you but there are times when it can be done. Think of 10 people searching a room, or a craftsman and 10 helpers building a house.
Sometimes the characters fail even when they have help. This rule seems to be designed to make it so that the characters cannot fail. To me this kind of defeats the purpose of the game and makes it into a situation where everybody gets a trophy. Without the chance of failure victory is meaningless.