I'm really excited for both roles and ideally would like to do both.
How do I persuade the company to take me on part time for a pro-rata'd salary?
If yes, it is possible, You will find somebody, who will work second half of work planned for you, and You will manage him and be responsible for these two half's.
Yes, real business questions, when somebody ask to do half of work, are, who will do other half, and who will manage this additional person.
If You could give strongly supported answer for these questions, You will hit.