The older DVRs have two tuners. If you have two shows recordings it will have to be on one of the channels because it can only physically tune two channels at one time.
The newer DVRs have 6 tuner so you certainly can record two shows and watch a third. Now if you already have a newer DVR you can just upgrade to Enhanced DVR. If you have an older DVR you will have to upgrade your boxes to FiOS TV One. What is your current DVR model?