Do you have a Turbo Scratcher? They are $20 at Petsm@rt and the like. My cats LOVE it. I don't know what it is about this particular toy--I'd think it was the catnip, but they've had other cardboard scratching boards with catnip inside and couldn't give a hoot. I think it's the combination of the catnip, the scratching, and the ball that rolls in the track...
Anyway, it's horizontal, which would simulate the carpet experience, but it might be more appealing, which would hopefully distract them from the carpet.
A couple of other thoughts:
-Get a sheet of that plastic stuff that goes under office chairs so they can roll on carpet (like this
from St@ples). Turn it upside-down so the "grippers" are facing up. Place it on the area that sees the most scratching. Cats don't like to walk on it, so hopefully they will avoid it. If they switch to another area for scratching, put the mat there. Keep moving it around to all of their preferred areas, and maybe they will eventually realize that they can't scratch the carpets for long without being thwarted.
-They may be bored during the day while you're away, which might contribute to this scratching. Make sure they have plenty of toys, like the Turbo Scratcher, and rotate in new ones when you can. Get one of those DVDs or videos made for cats. It sounds expensive, but it's probably cheaper than replacing your carpets, right?