The gemara says that if 2 people made a deal, reuven agrees to watch shimon's car and shimon agrees to watch reuven's coat, it is considered shemira b'balim. Therefore if either the car or the coat gets damaged, neither one is responsible. However, Rashi explains that this only applies to the case of שמור לי ואשמור לך, but in a case where they are borrowing from one another i.e. reuven borrows shimon's car and shimon borrows reuven's coat, it is not considered shemira b'balim and each one is chayev for any o'nes that would happen to the item. The definition of שאלה בבעלים is that at the time when the borrower borrows, the lender is doing work for him. When the lender is watching his item that is considered the lender doing work for the borrower, but if the lender is borrowing an item he is not considered to be working for the borrower.
The rationale for this distinction is that there is a fundamental difference between a borrower and other types of shomrim. A borrower is chayev on o'nes because כל הנאה שלו, all the benefit is his. Whereas all other shomrim are not receiving all the benefit. Therefore, a shomer chinam is certainly working for the ba'al habayis. Even a shomer sachar and perhaps a renter are considered to be working for the ba'al habayis, jus that they are being compensated for their work. However, a borrower is not working for the ba'al habayis. Actually the opposite is true, the object of the ba'al habayis is working for the borrower. That is why any other shomer is considered to be working for the ba'al habayis to create an exemption of בעליו עמו במלאכתו if the ba'al habayis would have borrowed something from the shomer, but a borrower is not considered working for the ba'al habayis so there isn't any exemption of בעליו עמו.