is using STP possible

I have two FG 60E devices.  I have a hardware switch setup with 2 interfaces on each one with ips of 192.168.21.1 and 192.168.1.23. Using one the hardware switch interfaces I have connected them over an ELAN circuit(layer 2) via ipsec tunnel and can communicate without a problem. I want to connect a second ELAN Circuit(layer 2) using the other hardware switch interface and use it as a backup in case of a down network. I have set the interfaces to use stp but it is causing a vpls mac loop and the provider had to shut the second circuit down.  Is this even possible or am I barking up the wrong tree.