I've been using my adsense account on multiple websites with no problems. You may want to look at how to set up 'site channels' in your adsense account so when you are looking at the reports you can tell the breakdown of which of your websites is making you how much money
Yes you can. I agree with devTastic. Setup channels to see which sites are making you money and try to improve those who are not making any.
err you don't need to create channels to use adsense on multiple sites, channels are to track which ad has been clicked