I am not aware of any specific limitation in regards to this configuration.
What troubleshooting steps have you performed to isolate the issue?
Do you see anything on the logs in either gateway that might provide some clues?
While mostly geared at larger gateways, many of these steps can be performed on the SMB appliances as well: How to run complete VPN debug on Security Gateway to troubleshoot VPN issues?
Retrieving data ...