Unlikely that your car came with 2 pot rears from factory, but they are a relatively common upgrade so it's always good to check. Only a select number of the GC8 STI models came with 2 pot rears as standard. Have a look through your rear wheels and see which of the following you have;

Single piston/pot caliper. Has the floating middle section and you can see the brake pad from the front. Most will be black/dark grey by now unless they've been painted.




Two piston/pot caliper. These came in red and black as standard, but may have been painted. If the Subaru logo isn't there, there'll be a flat rectangular surface where it should be.