Someone smarter and more experienced than me will hopefully address your second question but the first one, I think, is purely to do with the software that you are using to create a DVD.
My currently quick and dirty workflow for turning movies captured on iplayer into DVDs is this:
1/ Bring the MP2 file into VideoRedo Plus, edit out the tops, the tails and the adverts, then export as an MPEG file (and that export - just like the editing - seems to take no time at all, just a few minutes). The new file is a little bit smaller than the original, of course.
2/ Bring the MPEG file into Ulead DVDWorkshop, add chapters (but no menus - unless I'm feeling fancy) and export as a DVD .iso image
3/ Use DVD Decrypter to load that .iso image and burn a DVD. I use DVD Decryter, rather than any of the dedicated disc-burning suites I have available (Roxio and a couple from Nero), simply because DVD Decrypter attempts to do nothing other than burn the DVD - so it's acceptably fast (I'll typically choose only to burn at 4x mind) and I know there is going to be no changes made.
Using the above method - and depending on the length of the movie - I'm pretty sure that I could often get two movies on a single DVD - just by bringing the MPEG file for each movie into DVD Workshop.
But blank DVD are wonderfully cheap to buy, so I've never even thought to try doing this - I can see no point in bothering to have multiple movies on a single DVD.