[Bring_Back_Shantz]

I would say tell the guy from Job A exactly why you have to reschedule.

Telling Job B you can't make it because you have to go interview with someone else is a good way to get shown the door.

However, telling Job A that you have to reschdule the interview because you made a commitment to another potential employer (make sure to be a little ambiguous about this being training for a job you've already accepted) will not sound nearly as bad. Hell they may even like the fact that you are honest and are following through with a previous commitment.