Can we use Forest Trust or ADFS to address administrative cost of disconnected AD users in Forest A with mailboxes in Forest B

Question

Scenario:

We currently have two domains under two forests ABC.com AND DEF.com each with their own exchange 2016 instance. We need to migrate mailboxes from ABC.com to DEF.com but we still want ABC.com users to logon to their desktop as ABC.com users while accessing their mailbox on the DEF.com domain.

Is it possible to utilize ADFS or forest/ domain trusts to mitigate the administrative headache of having AD users and their mailbox disconnected in separate AD's. We do have site-to-site VPN in place.