How to merge several DVD's to one ?

Hello,
was not successful to find a similar thread, so :
How to to put several DVD's to one :
1. having a movie split to two DVD's with a lot of add-on crap on the first one, so DVD2ONE could easily make it fit.
2. having very short movies, where two of them would fir on one DVD

I don't have a clue about IfoEdit, so any guidance would be great.
Thanks

GG