You can't.
You can only have 1 VIDEO_TS folder per dvd.
You CAN have more than one VTS within that folder.
Import the VTS_01_1.VOB and join the rest in DVDLab Pro.
Import the VTS_01_1.VOB from the second movie and join the same way.
Reauthor.
Compile.
Shrink if needed.
Burn.

I don't know of any way to do this with two complete dvd titles though, unless a combination of Shrink and DVDRemake (maybe IFO Edit) can do it.