Every soft sleeper I've ever been on in China had 4 berths per compartment, including the Beijing-Xi'an train (although that was in 1998). I think you have to get the "te-ruan wo" ("special soft sleeper") if you want only two beds per compartment--as opposed to the regular "ruan wo" (soft sleeper). Or something like that. But it's definitely not available on most trains.

Whatever guidebook told you that soft sleepers would have only 2 berths is a bad guidebook--unless they meant two levels of bunks as opposed to the triple-decker bunks in the hard sleeper.

But we're getting a bit off-topic here.