I pay my seconds $25/hr., but they use my equipment. If someone had their own gear, I'd probably top out at $35/hr. We have the option to actually pay for a second photographer (otherwise they are not guaranteed one), in which case that whole fee goes to the second. Yeah, our studio is getting a few grand per wedding, but it's not like we just stuff that in our pockets. It pays for a lot of different things, including myself as the owner of the business. Gear, rent and utilities, computers and software, professional fees for lawyers and accountants, etc.
The amount any professional charges should have been calculated according to the costs of running the business--including paying themselves a wage to match or slightly exceed the cost of living. Assuming your employer has accounted for a second shooter in that calculation, it's probably a fixed amount per package or hour. Suddenly asking for another $25/hr. and not budging on it is probably a good way to get canned. Unless you are an incredible photographer.