Interesting question. Soyuz capsules are rated for 6 months on orbit. To have two capsules attached to the ISS at all times, you'd probably want to rotate one every 3 months. That would require doubling the Soyuz production and launch rate, which may not be all that easy since the Russians have apparently reduced the vehicle production rates. Then again, that might not be an issue at all. Would the Russians have enough trained Soyuz crews to support all of those missions? Probably, and they might even have the side benefit of being able to fly more tourists.

There's an issue of available docking ports on the ISS. Would you be able to have two Soyuz capsules docked and still receive regular Progress resupply missions and Shuttle missions? This might well be the bigger issue.

Personally, I'd prefer to see someone build the CRV or a comparable vehicle, such as an enlarged Soyuz capsule capable of carrying more people on an emergency reentry. I wonder how difficult it would be to create a Soyuz TM derivative along the lines of Big Gemini. It would require a larger reentry module, which would either mean either going with that new Soyuz booster upgrade that's being developed or reducing the size of something else, say the orbital module.