Can I create 2 x P365 Projects as below?
Project 1: Tenant A and tenant B, using @tenantb.com for address rewrite SMTP domain
Project 2: Tenant A and tenant C, using @tenantc.com for address rewrite SMTP domain.
Then if external user no matter sends to @tenante.com?
It has not been tested and is not recommended.
There could be some problems with the Opt-in Groups synchronization since they would be named the same. You would also have to be very careful with managing the users in each Project since they are the same source users in total. And decide which project will configure free/busy, will manage GAL Sync and Group Sync, only one can.