I doubt any program is going to import more than one menu and mix and match them...
ConvertXtoDVD does take multiple DVD files and puts them on one dvd. I have not tried to do this, but it works great for the AVI to DVDs. Have not used the menu funtion. The only limitation on the trial is a watermark.
For my own curiosity I wanted to see what the watermark looked like. Depending on the video size, the watermark can be bigger or smaller:
I just used TMPGENC DVD Author 3 to put two DVDs onto one. I made custom menus.... I'm pretty impressed with the functionality (first time). I found it a little hard, so you may find it pretty hard. Although I got real into all the advanced tweaks instead of using the templates.