It's just that you haven't decided about your feelings for each of the two. If you don't want to lose any of them, you'd better date both so that neither Sam, nor Janice know. That will be hard, i know. Have Janice ever told you she loves you? Or she wants you back just because she knows you met Sam?
Try to be frank with yourself. Who you need more? Anyway, the most effective method is taking a short break to think it over... Just tell them both you need some time to make decision...
Personally I would never date a guy if I knew he goes out with other girl.