I believe I know the answer to this, but figure I'll ask anyways:
I have 2 O365 tenants, Tenant One has @domain1.com and Tenant Two has @domain2.com.
Is there a way to enable a user in Tenant Two (email@example.com) to be able to send email as firstname.lastname@example.org from their outlook without having 2 accounts (one in each domain)?
Nope. Best you can do is verify a subdomain in the other tenant and use it to assign aliases and send messages. Or send via an external system, but make sure to take care of SPF/DKIM checks.