How to merge 2 Exchange 2016 organizations

So, compB's MX points to compA's organization; configure compB as an internal relay domain in compA, en configure a send connector for the respective domains pointing to compB's endpoints. If you have mail-users post-migration in compB (eg mailboxes moved from compB to compA), make sure you set targetAddress to a domain uniquely defined in compA (eg. when rebranding set to compA address). This, to prevent you having 2 locations with disjoint populations requiring internal relay for that domain pointing to each other (with bouncing NDRs). Alternativelt, introduce a mail routing domain - similar to Office 365 - such as mail.compb.com, which is an accepted domain in compA, internal relay in compB and set targetAddress in compB to those entries.