The Zune Marketplace subscription will allow you to sync two different Zunes and two different computers with the same account. What I found out by accident is that you can actually use a completely different brand of player for the second device. You may not even need a Zune; you may be able to sync two non-Zune PlaysForSure MP3 players. I have only tried this on one account so I may need some of you to further verify this. So please let me know if anyone else is successful.
I did this on an updated Vista computers using the Zune software and Windows Media Player 11. What happened was I connected a bunch of MP3 players to charge them and by accident my Sansa View started synching the library automatically though WMP. WMP saw my Zune subscription folder and authorized the Sansa View to download the tracks as the second device. I would also imagine that you could not use a Zune at all, use the Zune Marketplace Subscription to download tracks and sync them to two PlaysForSure MTP based players.
Now if we can only get the Zune Software to recognize other MTP based players or get the Zune on WMP11, we could crack that pesky ecosystem open.