Hello,
If I read your message correctly your Second Ironport does not have knowledge of what recipients reside on Exchange1 or Exchange2. It simply routes all mail to @test.com to Exchange2.
This would explain your NDR's, since this means a mail to @test.com from Exchange2 will be routed back directly to Exchange2.
There are (at least) two possible solutions for this:
1) Give Ironport2 knowledge about where to route mail to (Ironport1 or Exchange2).
2) Create a policy that forwards all mail received from Exchange2, targeted to @test.com to Ironport1. (If you want, I can assist you in creating this policy)
Good Luck!
Steven