You are the best person to answer your own question.
If I add 2 x avi movies, and end up with 4 hours of video on 1 DVD, do YOU like the end results ?
I never personally crossed 3 hours, but some people say up to 4 hours it’s still very good.
Then again, you have some very picky users who even complain with 1 hour of video
I would say, get some RW media and give it a try.
Of course, both input must me of great quality, since ConvertXtoDVD will provide you the best it can based on the input file.