you can do both. What you can not do is bid on the same keyword for different accounts but with the same URL.
I would not advise promoting the same url. If you will promote the same url, then use one account at a time. Once the credits run out, delete the campaign and remove your site url. Then proceed to your next account and repeat process.
I have 12 different adwords account running through the same computer. It's been like that for about 3 years now. One of my client, I´m running his campaigns on different accounts, but they are the same URL, what I have different, the keywords just not to piss google off