If you fall behind it won't look good on them not you. You don't have to cover your back, they have to cover their back. The onus is on them to explain to you how you will cover two job roles under the same contracted hours.
Tell them you expect it reviewed after a month so you can discuss if its working or if you are struggling to manage.
Suggest sharing the person's duties out between more of you rather than just you.
The most important thing is do what you can without over stretching yourself of stressing yourself out. If they complain about you falling behind on tasks say its because I'm working the same hours but doing two jobs.
If its not manageable its not your responsiblity, its their problem to sort out a better way of covering the extra tasks.