Combine users and groups from different AD domains

Problem Definition:

We have two user directories whose domain is different from each other. Their configuration for example as below.

Directory 1:
server: cn1.xxx.com
base DN: dc=c1,dc=xxx

Directory 2:
server: cn2.xxx.com
base DN: dc=cn2,dc=xxx

A group called Maintain Group is in cn1. Let's say four users in this group, both user A and B are in cn1, and user C and D are in cn2. Although all of them are synchronized into Confluence, but only A and B are under Maintain Group. User C and D are not memeber of this group.

AD cluster is not supported by the Confluence server or data centre solutions.

The only solution we have here is adding the groups to the same server as the parent group.

Suggested Solution:

Different groups from different AD can be combined based on our structure.