Companion control in co-op being bugged is a known issue and one that is going to be addressed (hopefully in this week's patch...cross your fingers).
For right now though, the only way to have the 2nd player control companions is to have him/her directly hire them. If a companion is already in the party, you have to dismiss them and then have the 2nd player hire them back. And once you reload the game, control of companions will go back to the host, so you have to do this whole song and dance again.
...Yeah, it's messed up.
