I'm not sure from the description, but it sounds like you have both firewalls the SSG and the SRX connected to the same server LAN segment directly. Thus the servers have to choose one or the other as their default gateway.
This setup will create asymmetrical flows in some circumstances.
Best practice would have only one gateway for the servers then connect a routed link between the second firewall and the first for traffic out to the second ISP.
If that is not possible, you need to identify all of the asymmetrical flows then add a source nat on the default gateway firewall on those flows. This will then force the other firewall to send the traffic back to the originating firewall symmetrically instead of having the traffic go directly between the servers at layer 2.