The easiest way to get two or more movies on to one DVD is to collect the files on a hard drive and when you have collected enough to fill a DVD, then is the time to write them there. I would suggest that you don't write to a DVD RW - they are not very reliable for permanent storage.
On the compatibility question, I've not used Roxio but I have mixed other editing software. Provided the files have been in a format that the software can understand, I have mixed several different programs during the processing of one "masterpiece"!!