New Domains in already existing O365 Instance and adding new instance in already existing Azure AD

Hi Team,

Our Company is going to split from existing company into two. We are having already existing O365 Tenant and we are planning to add extra domain in the same existing O365 Tenant. 

Also we are planning to maintain the same single instance in Azure AD and create two instances (Two different UPNs) for two different companies. 

Note: We need to consider both companies as two different entities 

So, kindly let us know the best approach to be followed and also let us know the Pros and Cons.

please let me know if any information is needed from our end.

Thanks in advance!!

Thanks,
Raj