WAN LLB and VPN site to site

Hi,

We have deployed fortigate 100D ( 5.4), currently we are using a single ISP with VPN L2L tunnels.

Now we want to install a second ISP for our internet access using WAN LLB, but my question is that how can we configure the existing VPN L2L ?  Do I need to configure them using the old interfaces ( wan1 and wan2) ? how can I route them ? the plan is to have active/stand by vpn tunnerls , our remotes sites have only 1 single internet connection.

Thank you.

Regards,

Luis C.