Ok, scratch that. Currently it's just my wife and our son, but we plan on a second child. So that will be four set-top-boxes (eventually)
At least there's one bit of hope: our son is only a few weeks old now, and his sibling doesn't even exist yet. So by the time they both will want a TV in their room (say at least ten years from now), I'm hoping there will be a solution for this problem.
I'm guessing all I can do in the meantime is put as much future-proof cable in the walls as possible. I might even put in fiber, even though I absolutely have no reason or need for it now at all. But I'm guessing it might be useful in the future...

I must say I find it really stupid and maddening that all this is possible without a problem with 40 year old technology (analog TV), but new technology (digital TV) simply can't do this. That's not progress in my book. I wouldn't even care about this for the smaller TV sets in the house (eg. in the kitchen), but I don't want to risk putting those solely on analog knowing that in a number of years in the future, analog TV will dissapear completely. I can't imagine that those digital TV manufacturers haven't considered the fact that almost everybody has more than one TV set in their house nowadays. If they did consider this and decided they didn't care, that's just plain negligence...