Have you ever been putting your deodorant on, get one pit all lathered up, and then the deodorant runs out? Let's say for arguments sake that you have another stick of deodorant, but it's a different flavor.
Now you're in a quandary! You have a decision to make. Do you just go with one pit covered and hope it's a cool day today? Do you wash your one pit off, and start over again with the other stick of deodorant? Or do you just use the other stick on the second pit and intermingle the different fragrances?
I suppose there is always the fourth option of washing your one pit and just going without deodorant altogether, but I wouldn't recommend this.