Try this as a thought,
On the first server, tell it to "forward all unresolved to.."
and put in the I.P. address of the 2nd server as noted here:[xxx.xxx.xxx.xxx] (the brackets are necessary for I.P. addresses), that is if you have a external I.P. address for the second box. see if that helps.
What that should do is allow the first server to receive all mail for your domain and then forward any recipients that it doesn't resolve to the second server. This will only allow for forwarding and not true inter-connectivity.
Wyz