Thanks for all the thoughtful replies!
I'm going to have a look at those Denon receivers that don't have this limitation and see how pricey they are.
I like the idea of only having digital sources going into the zone 1 receiver, but all my sources can't do digital (I have a VCR).
The source I definitely need in zones 1 and 2 is the Apple Airport Express. And this is the only source that cannot output digital and analog at the same time. Like all the new Macs, it has a single mini-DIN (I think?) audio output jack that outputs analog or optical digital depending on what you plug into it.
I like the Squeezebox, but I really like using iTunes on my laptop to control what's playing. The Airport Express is simple and works best for me. Most of my MP3s are encoded at at least 256K variable bit rate, so trying to go digital might be worth it.
So what if I got a digital optical splitter cable to plug into the Airport Express? One split would go into the zone 1 receiver's digital input, and the other into a separate D/A converter? Then I could go from the D/A converter either directly to the zone 2 receiver or to analog inputs of the zone 1 receiver.
Are there reasonably priced separate D/A converters that would make this a reasonable solution?