the question is will dual S3600i's give me more than what i lose with a single V3600i?
In total, I'd say yes. If you look at PSA's self-reported CEA numbers, yes the V3600i still has a marginal advantage over even a pair of S3600i's in the 16-25Hz range (122.4dB vs 115.1dB x2). However, that kind of simple analysis ignores a few key factors.
As you mentioned, there's the matter of extension; assuming some modicum of room gain, a pair of S3600's should net you a respectable amount of output down to 10Hz or below. The corollary to this is that such a setup would be harder to bottom out as well; below the tuning frequency of a vented design, excursion rapidly goes off the charts. In addition, ports tend to compress and chuff as they approach their limits. With respect to compression, what that means is that as you approach the limits of a ported system vs a sealed system with comparable burst output down low, the sealed system may actually be producing more output. It's also worth keeping in mind that many sealed systems can produce low end output in excess of their CEA burst rating, but with higher distortion. Depending on the makeup of that distortion, it may be relatively benign and go unnoticed.
Finally, there's the advantage of having double the drivers and double the power to keep in mind. On the face of it, it doesn't matter because you're probably thinking you don't need more output on the top end. However, what that additional output implies is that the system is using fewer resources (namely percentage of amp power) to produce any given level in the 50Hz+ range, leaving more on tap for producing the deep stuff. Given that we listen to complex content vs CEA bursts, the impact of this shouldn't be underestimated.
To put it all in practical terms, I'm not pining away for my old PB13, even though it technically has significantly more 20Hz burst output than my current FW18.
