STP between VLANs on the same switch

Obligatory diagram: http://www.gliffy.com/pubdoc/4490946/L.png

I have a pair of Exinda traffic shaping appliances. Each has their LAN and WAN interfaces tied together in a bridge pair. The inside interfaces are connected to the core via a three port VLAN (704) on a switch. Likewise, the outside interfaces are connected to our border router via the same setup with VLAN 705 on that same switch.

I know I need to set up STP to ensure that only one of the paths is active at a time, but I'm not quite sure how to go about it. Do I set up one instance for the ports in 704 and another instance for the ports in 705?

Thanks!

UPDATE! OP DELIVERS! - Ok, so a single switch just wouldn't work. As someone pointed out below, the root bridge will always have all ports set to FORWARDING. I just ended up provisioning another switch and doing it "the normal way". =)