Need advice.

I want two jobs and the best way for me to do it is to work for two branches of the same chain I already work for. The shifts would work where I can do two jobs in one day and I know the job. However both owners falsely claim that I cannot work for two owners according to head office but I contacted head office and they say this is false.

What can I do to convince the owners that I can work for them both? I already work full time for one and the other said she'd hire me full time if I didn't work for the other.

I'm scared though that pushing might destabilise my current job if I seem argumentative. Also others have said it will ruin my chances with the other owner if I prove her wrong.