Assuming based on the question you are running ISE VMs. Add a new adapter in VMware. Open ISE cli for the node you want to add/configure the NIC. Typically the first NIC used is g 0 and your next one will be g 1. Go into global config, int g 1, and configure your ip address for the other subnet and no shut the interface. You will probably need to add static routes to send your guest traffic to g 1 gateway instead of default. To tell the portal to use the other interface, you configure this under portal settings (work centers->guest access->portals&components->guest portals->portal settings. Select the interface you wish to use.
Some things to consider: you may need to use static IP in your authz profile for this new interface/ip for cwa redirect. Also, make sure your default acl_webauth_redirect acl has the proper configuration after change so that hosts that should be redirected to the portal are permitted and can get there.
HTH!
