It has been my experience that if there are timer conflicts, then one of them will get skipped entirely. If a tuner is in use watching or recording live TV, then you get a pop-up asking what you want to do, stop recording, or stop the timer. If no one is there to answer the question, the timer will override the live tv viewing/recording. In other words, in my setup, back-to-back timers on the same channel will always attempt to use the other tuner...if it can't, then the timer with the lower priority will get skipped.
As far as PTAT goes, since the entire TP stream is being saved to another partition, I would hope that same-channel overlaps are not a problem for a single tuner, just as being able to save 2 or more programs from different channels from the same time slot.