Adding a UPN Suffix that is already synced to O365

Hello,

We have 2 forests: one.org and two.ca; Both are synced with Office365 in a single tenant.

We would like to add a UPN suffix called two.ca to the one.org forest - will this cause any issues since that UPN suffix is already synced to Office365 and assigned to another forest? Is there any other way to achieve this?

Thanks!