Good points above. Just be careful you don't disqualify due to the fact that they are unmarried, that is discrimination in FHA eyes. The other questions I would ask are:
1. How long have they been employed (I use 1 year of stable income)?
2. Is their credit report good/clean?
3. Talk to their current landlord and one before that and ask questions about if they were ever late, ever break terms, leave the place nice etc.
I would be very careful of throwing out their application, you need to have very specific criteria and guidelines that you need to follow for every applicant. If they don't meet the criteria then you have documented reasons for denying their application. Generally its safer to process applications first come first served, so make a decision on this couple then move onto others..
This is just my opinion, I have just done my first tenant turnover and it was enlightening. I learnt a lot about being very careful on how you qualify/disqualify applicants by using my criteria (and a scoring system-I think its makes it very clear cut, if they score X then they qualify, if they don't score X you can give the opportunity to score higher with a double deposit for example, except if there is an automatic disqualifier such as past eviction).
