I take it you want to combine two small dvd videos into one, not just merge the vobs. Merging the vobs is slightly different, and there is a tool called VobMerge, which will produce one very large vob file from a list of them, but the result won't be compliant to dvd-video standards anymore.
Here is what I would do. I would take your video out of the .vob files and combine them into mpeg files. This is easy to do using Vob2Mpg, and will not degrade the quality of the video. Do this for each dvd that you have now, so if you started with 2 mini-dvds, you should get two separate mpeg2 files.
Now, import those mpeg2 files into DVDFlick. This tool will let you make a menu so that you can select either video. It will produce a finished dvd, ready to burn to a disk, and you can even set it to burn automatically if you wish.
Make certain to set DVDFlick to only copy the mpeg2 videos when it makes the dvd. This way you won't lose quality by reencoding. You do this by clicking on Project Settings-->Video tab-->Advanced. Put a check mark in the box next to Copy Mpeg2 Streams, then click Accept.
You can find Vob2Mpg here: http://www.videohelp.com/tools/VOB2MPG
DVDFlick is here: http://www.dvdflick.net/