The gemara says that if partners steal they are both chayev, but if reuven asks shimon to steal for him, only shimon is chayev. Rashi explains that the case of partners stealisn is when one partner does the act of geneiva for both. Since he does it on behalf of both of them, they are both chayev. Tosafos disagrees because we would only says that one can be a shliach l'dvar aveira on the act of shechita to be chayev 4 or 5, but on the act of geneiva even if reuven and shimon are partners, we would not say that shimon can serve as reuven's shliach to make reuven chayev for geneiva. Therefore, the only case that reuven would be chayev is when he commits the act of geneiva together with Shimon.
The Maharatz Chiyus points out that this is difficult even according to Tosafos. Why would we say that when reuven and shimon pick it up together - מיגו דזכי לנפשיה זכי נמי לחבריה. Even when they steal it together, since we have a concept of אין שליח לדבר עבירה, reuven is not able to serve as shimon's agent to steal it, and shimon cannot serve as reuven's agent, so it should be considered from each one's perspective as if the other half of the talis is still lying on the ground and they were never koneh it? The mahartz chiyus explains based on the sm"a (c.m. 182) who says that אין שליח לדבר עבירה is based on the idea that the sender doesn't really expect the shliach to do the issur, because he knows that the shliach should listen to Hashem rather than him. This only makes sense in a case where the sender maintains his innocence, but in our case where both reuven and shimon are doing an act of stealing, they aren't innocent and cannot claim that they didn't expect the other person to follow through with the shlichus of an aveira. Therefore, in the case where reuven and shimon are doing the aveira together we say יש שליח לדבר עבירה and each one serves as the agent of the other to help him be koneh.