No, not by showing both cards. I wouldn't even try to do it. This will probably not impress the lounge dragons at all. I think it is better you ask them politely. Maybe you are very lucky and you have a nice lounge-dragon and the lounge is not very full so they let your whole family in. Otherwise the rule is one guest only.

The only other thing that might work is to buy two day passes for the RCC Frankfurt but I don't know if they even sell them in Frankfurt.