From the Migration Manager Install Guide, page 11 - "Establishing Trusts"
Trusts between the source and target domains are not required for Active Directory or Exchange migration with Migration Manager. However, we recommend that you establish two-way trusts between each source and target domain that will participate in migration.
If the forest functional level in both source and target forests is set to Windows 2003 or higher, you can establish forest trust between the forest root domains.
Trusts make it possible to resolve objects’ security identifiers (SIDs), which in turn helps to distinguish objects and check that everything is going right. Trusts also help provide co-existence of two environments, including uninterrupted access to the resources for both switched users and users not yet switched.
For more information about the restrictions that apply and possible migration scenarios without having trusts established, refer to the Migration Manager Best Practices document.
We recommend establishing two-way external trusts between each source and target domain that will participate in migration.