Hi partner,
Just one sentence description may be ambiguous. Let me explain in detail.
At the beginning, there are two business units BKTest1 and BKTest2, and users of BKTest2 could not access Leads of BKTest1.
Therefore, we need to assign the permissions of BKTest1 to the users of BKTest2.
Then, we create an owner team of BKTest1 and a security role of BKTest1. Assign this security role to this team.
Finally, we add the users of BKTest2 to this team. In this way, BKTest2 users can access Leads of BKTest1.
If you want to allow BKTest1 users to access Leads of BKTest2, just create a BKTest2 security role and team, and add BKTest1 users to this team.