tcp outgoing ip address in isa 2006

i installed isa 2006 on windows 2003 STD ,
i have 2 ip address on this server , one for my clients and second ip is empty !
now i want to use vpn for my persoanl usage , but its using server first ip that shared between clients ! 🙁

So i want to know how can i configure isa 2006 that send/request data using my second IP address