I generally would advise against this practice only because you are accepting responsibility for your co-workers bag.

If you were on the same PNR, your co-worker would just inherit your benefit.
If you are not on the same PNR, I don't know the policy. I know they typically will allow you to designate a companion for things like E+ so I would assume baggage as well. YMMV I would assume. The agent may extend the benefit anyways.
Certainly, they can do it when a PNR gets split but thats means they were together to start with.

Generally, on company travel, I don't make too much of an effort to save money like this. It all gets expensed anyways and its better to capture the true cost if they are using actual spending to estimate future projects.