My boss wants me to do my job and someone else’s at the same time since they’re quitting. Since I’m unionized they can’t personally give me a raise what should I do?

Pretty much what the title states. I work a job that requires a full 8 hours a day to finish and my boss wants me to do my and my (now ex) coworkers job in the same amount of time everyday. I’m unionized so he can’t personally give me a raise to incentivize the work so what do I do? Tell him no? Ask for a raise anyway? I don’t feel I should have to do two peoples work just because they quit. Any advice?