Did a box open when loading the AVI into VirtualDUB talking about VBR Audio? If so, then use Nandub to join them. VirtualDUB cant handle VBR MP3 or AC3 audio.
If the 2 AVI's are equal length time wise, then encode each separately to 1/2 DVDR size, ie set the CD Size in Bitrate tab to 2220 rather than 4440. Then author in DVDLab and link the end of the first movie to the start of the second.
Please disregard the Preview in DVD2SVCD, it sucs big time and I make no reference to it in the tutorial. It never shows correct aspect ratio 99% of avis/movies are now 16:9 so select the anamorphic option. If its a full screen cartoon, then select the 4:3.