If your questions are to be included in the final edited program then 2 mics would be best. One facing the interviewee and one facing the interviewer, each on separate channels.
If you just want to hear your questions for yourself and are editing them out then a single omni interview mic would be fine. The single mic would be facing the interviewee with interviewer close enough to be picked up by it but don't speak over the subject.
Typical single mic interviews work by the interviewer standing/sitting next to interviewee, the interviewer controls the mic holding the stick mic and pointing it at each talker in turn.
Omnis are typically used for interview mics as they are less susceptible to wind/handling noise and will pick up some of the surrounding atmosphere/ambience. They are also more forgiving of positioning as you are moving/pointing the mic during conversation. If you interviewing in a noisy environment the mic will need to be positioned closer to the mouth of each speaker and in extreme situations like sporting events or factory floor etc. cardioid mics would be better suited.
Bear in mind it can be beneficial to record some atmosphere/ambience separately while on location, which could be useful for smoothing out your edits and the sonic flow of your interview.