The reason you want to bump your credit line is to increase your credit score for when you want to buy a car or a home. The first credit is easy. It's the second. And you do need two cards. Sometimes the first card is having transaction problems (computer, etc). I recently had one frozen because of suspicious activity. So I had to use another card.
To get another card, just wait until Xmas. Go somewhere like Best Buy, select your goods, and ask for a credit application. If it gets rejected no sweat. Pull out your exisiting credit card and charge. By the time you get home, you'll have a pre-approved in the mail!