A good solution could be to add 2 manual Balancing Rules in the Net Balancer. One of this rule with the source address the subnet 192.168.1.0/25 and the first gateway. The other rule with source IP the subnet 192.168.1.128/25 and the second gateway. So all the clients with IP in the range 192.168.1.1-192.168.1.127 will use the first gateway and the others with IP in the range 192.168.1.128-192.168.1.254 the second one.
Regards
Fulvio