I currently have two mountain bikes (a rigid singlespeed for round town, quick blasts, red trail centre routes and muddy rides) and a full-sus for XC racing, remote rides, and anywhere I think a rigid singlespeed would struggle. I could get by with one, but I'd be chewing through drivetrains, and the forks would need serviced far more quickly on the FS. I can also wash the singlespeed in about a third of the time it takes to clean out the suspension bits and gears on the full-sus, and I'm slightly less worried about leaving the singlespeed chained up to railings. I'd say that on average, the singlespeed gets ridden 2-3 times a week at the moment, and the full-sus goes out once every 3 weeks (more in the summer).
However, if I had a CX bike, I could probably get away without the singlespeed. I'll get shot down in flames for this, but I do think that too many bikes can be a disadvantage, and just means that you have far more work to do to keep them running. If you head out on one bike on Monday night, and the other on Tuesday for example, and discover that the front brake rubs slightly on both, you now have two bikes to fettle with on Wednesday.
Back to the original question, if you have a middleground MTB (do it all hardtail or full sus) and just ride it at weekends, I reckon you could happily get away with one bike, provided you aren't using it for everything from XC racing to downhill.
