configuring load balancing on RB1200

hi, i have a RB1200 configured for me by a friend, 2nd port configured for DHCP 192.168.100.0/24 and internet on the third port. now i have another internet connection and i want to configure it so that only 192.168.100.200 - 240 connect throuth the second interface and the others connect through the new connection, also the second internet connection is static so i have to assign an address to the interface i plug it in to.
any help would be much appreciated.
thanks