Steelhead appliances are not capable of optimizing any encrypted traffic between VPN concentrators, but it is of course capable of optimizing traffic coming out of the VPN concentrator.
A common problem is that the Steelhead appliances are setup on the network-side of the network, while the VPN concentrators are setup on the server-side of the network.
In this case, the traffic from the hosts behind a remote VPN gateway towards the servers will not be optimized because the non-VPN traffic towards the servers does not touch the Steelhead appliance. Traffic towards the servers across the WAN will be optimized though.
The correct location for the VPN concentrator should be between the Steelhead appliance and the firewall.