A lot of times, if two back-to-back episodes premiere at once (especially if they are two-parters), they're presented as a full-hour, with the opening and closing credits only shown once, and then in reruns they are normally split into two (though Family Guy has had hour-long episodes that always air as a full hour on Adult Swim, though they probably split the half-hours in syndication). I wonder if DVD releases normally present such episodes. But I think the two halves are normally produced separately (I've seen that confirmed for Garfield and Friends, which was broadcast as an hour-long show for most of its original run on CBS, excluding the first and last seasons). And often when two episodes are presented as an hour, the opening credits (at least for the original broadcast) note separate writers/directors for "part 1" and "part 2". But in recent years there have been times when two new episodes of a show aired back-to-back without the whole hour being presented as one. I recall How I Met Your Mother did that once, can't remember which two episodes they were.