Ask HN: Validating 2 Ideas in 1 Survey

I have a few questions about validating ideas in a survey. I'd like to use mechanical turk + a survey site to validate my idea. Do sites like mechanical turk give you info on a responder or do you have to ask for demographics in the survey itself? If I wanted to validate the combination of two ideas combined, would I put all of the questions in one survey, or break out it by idea into 2 surveys? I'm trying to not lead responders into answers. Would the data even be valid if split between surveys?