As with iPods, you can have multiple Apple TVs synced to one computer. You can't have multiple computers syncing with one Apple tv (although you can stream, but that's a different story).
The only restriction in your scenario is that if you buy content from iTunes, you are restricted to 5 devices.... And, if you're renting movies from iTunes, note that you can have each money on only ONE device (your computer, your 1st Apple TV OR your second Apple TV)...Of course, you can transfer the rented movie from one ATV to the other, but it won't appear on both. I'm guessing that's not an issue for 99.99% of people though.