Missing values and descriptives

I would like to combine the responses from two questions that answer to the same query. Both are from the same population (5000 subjects). The first one has 130 positive answers and 200 missing values and the second one 570 positive answers and 450 missing values. How can i combine these responses in order to calculate descriptive statistics. The total number of subjects that answered positive is 590 (some overlapped). I want to calculate the percentage but which is the new population and how can i treat missing values. Sorry,but i am a little confused.