Two pass encoding is only useful when you are using variable bit rate encoding (VBR) in order to fit more than about 72 minutes of video on a single-sided DVD and need to use a bitrate lower than a CBR of about 8Mbps.

Let's say you were trying to fit 2 hours on a DVD (using 192kbps .AC3 for the audio), that would require a video bitrate of about 4,896,000 bps in order to make it fit on the disc. You could encode the whole video at that bitrate, but the more effective method is to use VBR encoding where the bits available are used more flexibly to encode the video data more accurately, with fewer bits used in less demanding passages and more bits used in difficult-to-encode passages. The average bitrate over the length of the file would stay at 4,896,000 for example, but could go as high as whatever maximum you have set (usually 8,000,000 bps) for those passages which need it. Of course, other less complex passages would get fewer than 4,896,000 bps in order to maintain the average.

By doing a 2-pass encode, the encoder analyzes the file on the first pass looking for scenes which need the extra bits and those that don't, on the second pass it does the actual encode using the bitrate allocation strategy it calculated on the first pass. 2-pass encoding also ensures that the encoder more accurately hits the average bitrate target for which it was set. The only downside to 2-pass encoding is that it takes about twice as long to encode a file. I can easily see the difference.

Thanks MusicVid & John for your insight and replies.
I did render a SD video last night using this option, and yes it doubled the render times as reported.
And for what it's worth I personally did notice a slight improvement on video quality.
I may put this as a last step in my workflow prior to distributing my videos for optimal palyback quality.

Ted