Have a look at the ISPC database: dbispconfig > server_ip There you can see the IPs connected with the serverid.
You create servers in WHMCS with those IPs. The module takes this IP in WHMCS and asks ISPC about the serverid.
Create two servers with those IPS. If you select s1, than all websites/files are on server1. If you select s2, than all websites/files are on server2.