uh... wtf... so are you saying you work out your ENTIRE body with 2 20lb dumbbells???
20 lb dumbbells will probably be too light to do any much of a workout to your chest either... You can't even afford a bench?! oh my lord. I think the best thing you can do is dumbbell flys on the floor and that would still not work that well. Just do push ups. I think that would be best