As I've understood you are speaking about ISP redundancy, because you want to establish the second connection to the ISP.
There are ISP redundancy configuration steps, check it, because you might be have done a little mistake:
1)Open GW or cluster object properties
2)go to the Other>ISP Redundancy
3)select support ISP Redundancy
4)choose the mode : Load Sharing or Primary/Backup
5)Adding ISP links:
1.in the ISP links in order of priority click Add
2.enter the name, interface,next hop IP (it's ISP's IP)
3. repeat 1-2 steps to add one more link.
4. click OK.
Also if you want to configure DNS proxy ro VPN with ISP Redundancy you needs an addditional configuration.
Hope that will help you.