584. Two People Performing a Single Labor

Shabbos 1:14

If a person wanted to perform a particular act of labor on Shabbos, he started to perform that labor, and he performed a sufficient amount of work to be liable, he is liable even if he did not complete the goal he set out to achieve. For example, if one wanted to write some correspondence or a business contract on Shabbos, we do not say that he is exempt until he finishes writing the entire document. Rather, he is liable as soon as he writes two letters. Similarly, one who wants to weave an entire garment is liable after weaving two strands. Even though he planned to make an entire garment, he is liable because he intentionally performed the quantity of work necessary to incur liability. This is the case in all similar cases.

Shabbos 1:15

If two people perform an act of labor together that either one of them could have performed alone, they are exempt. This is so whether one performed part of the labor and the other performed the rest, such as if one of them removed an object from one Shabbos domain and the other placed it down in a different domain, or if they performed the labor jointly from start to finish, such as if they both held a pen and wrote with it or if they both held an object and transferred it between different domains.