It could be for 2 reasons. The original movie was on 2 disks, or else to maintain quality they've spread the movie over 2 disks (cd size.) (One of the disk might have even been the xtras.) I'd say chances are they're SVCD quality spread onto 2 disks.

If you're burning to VCD then probably be best to burn to 2 disks unless you wish to join them & compress them further in size down to one disk. But that would reduce the viewing quality significantly. If you are burning to dvd then you can have one as track 1 & the other as track 2 on the disk. Else you can also use a joiner utility.

Here's an example of a joiner: http://jfuse.com/ but this is just one & I don't know anything about it or how good it is. There's many out there & I think divx pro can even do it.