Malchiang, Not sure about that. But, why would you want to? The reason for having separate accounts (at least one reason) is your reputation. If you have a poor performing or great performing account, you might want to avoid using the same one. If you have a great performing account, you can start a new campaign in another account so you do not spoil the first one. On the other hand, if it is poor performing, you can try a different campaign to get a good performing account.
if your current adwords account is in good standing, then yes you can. lot of advertisers use multiple accounts as google limits your daily spending on each campaign. once way to get around that is to use multiple accounts
Yes you can open multiple accounts under the same credit card. Yes you will get caught if you're opening multiple accounts using different credit cards...eventually. So if you're in it for the long term, don't try to cheat them for a lousy couple hundred in free clicks. You'll get caught eventually and then come back here and claim they banned you for "no reason".