Hi Tony,
When you migrate the two domains to the third domain, the security groups with the same name will be merged into a single group in the new domain. However, it is important to note that the group membership will not be merged automatically. You will need to manually add the members of the old groups to the new merged group in the new domain. Additionally, you will need to update any references to the old groups in SCCM collections to point to the new merged group in the new domain.
Best regards,
Qiuyang