Site 2 Site VPN Gateway Source

I have two ASGs configured with S2S IPSec VPN. I am not using automatic firewall rules because I have different trust zones on each side that need to be able to talk to their equivalent trust zone. I have a RADIUS server on Side B of the tunnel that needs to be used when authenticating via L2TP VPN on Side A.

The request from the ASG on Side A is coming from a lower trust zone gateway interface (lets say 192.168.2.1) which is not allowed access into the higher zone on Side B. How do I force all requests from the Side A ASG to be sent from the higher trust zone interface (lets say 192.16.1.1) across the tunnel so it is permitted into the equivalent trust zone on Side B.

Thanks in advance!