Sometimes the party wants to complete a multi-part task where the performance of one of them on one part can help or hinder the performance of another one on a different part. They have a common goal, but are involved in separate and discrete parts of the task.
For example, in one game I DM'd, an invisible party member entered in a busy public square and needed to attract the attention of another party member. They knew the second party member was close at hand, but did not know exactly where they were. I had them roll Intimidation(Charisma) for how loud they could shout, and had the other PC roll Perception(Wisdom) for how well they could pick out the shout against the background noise of the city. With the overall goal being to convey the message, the better the first PC performed on their part, the easier the part of the second PC should be.
Now, I could have just had the second PC roll Perception with advantage due to the assistance of the first PC, as in the rules for Working Together. However, this was unsatisfying to me, because it would have the first PC provide a 'static' bonus without accounting for how well they did on their part of the task. It also goes against the requirements of the 'Working Together' rules themselves, which state:
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task.
Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.
Even though the PCs had a common and interdependent goal, the first PC can't really help the second one listen, and the second one can't really help the first one shout.
I could also call this a group check:
When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren't.
To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails. Group checks don't come up very often, and they're most useful when all the characters succeed or fail as a group.
While the PC's certainly are trying to accomplish something as a group and those who perform better are 'covering' those who do not, this misses the point by having everyone involved roll on the same skill when in my example they are using completely different skills to achieve different parts of a task.
At the time this event played out, I just had each of them roll, with the result of the first roll vaguely informing me of the DC for the second roll. I would like something a little more formal, however.
I am thinking that what I could do is set an 'overall' DC for the task and double it. The result of the first PC's roll would then reduce the DC of the task for the second PC.
For example, if my overall assessment of the DC for the task (hearing the shout) was 'Moderate' (DC15), then the combined DC would be 30. If the first PC rolled a 10 for their Intimidation attempt, that would reduce the DC by 10, such that a not-very-impressive shout meant that the second PC needed a DC20 Perception check to hear it.
I'm interested in an evaluation of this proposed mechanic. If you think this type of situation is covered in RAW, then please explain how. If you think it is not, please keep in mind good subjective as applied to homebrew review questions - a good answer should describe actual experience with trying to accomplish the same goal that I have, even if the mechanic to achieve it was different. Also, an objective assessment of the numerical consequences of such a mechanic (how it compares to a single skill check, to the 'working together' rules, to 'group checks', and how it is different from 'rolling to failure') would be welcome, even absent experience in implementing something like this.