I think there actually are dual fire tubes in the back, they're right at the base of the "scoop" in the hull, where the Galaxy has a tractor emitter. I don't have access to any pics at the moment, but I think that little box has two tubes.
Actually though, I'd need to watch Insurrection for that one..I think that's the only time we see the FC style Sov firing rear torps.
I know in Nemesis, there are several rear torps.. two above the bridge, one above the rear shuttle bay, one under the shuttle bay... and on the CG Sov, I put in the two at the bottom of the hull. At the time I was building it, everyone was more concerned with the forward weapons, so I think those didn't get scrutinized much..now that I think of it, I'm not sure I've ever seen confirmation of those two.