Another piece of information:
From Server_1 in Domain_B, issuing the manual commands for [$certrequest and $certadmin] while specifying the known CA and CN names of the two CAs in Domain_B, both command sets will complete successfully.
From Server_1 in Domain_B, issuing the manual commands for [$certrequest and $certadmin] while specifying the known CA and CN names of two of the CAs in Domain_A, both command sets complete successfully.
From Server_1 in Domain_B, issuing the manual commands for [$certrequest and $certadmin] while specifying the known CA and CN names of the two CAs in Domain_B, both command sets will complete successfully.
From Server_1 in Domain_B, issuing the manual commands for [$certrequest and $certadmin] while specifying the known CA and CN names of two of the CAs in Domain_A, both command sets complete successfully.