It would be acceptable to ask whether an individual was terminated or asked to leave a position in the past two years, and it would be acceptable to ask for the reason. The only problem I can see here would be the very rare situation where someone was terminated for a reason like filing a discrimination complaint or failing to return from a medical/maternity/military leave. This situations should be handled very carefully. One other question you might want to consider: how will you handle situations where an individual was terminated as part of a reduction in the workforce?