Hmm, that's an interesting scenario. The underlying engine has the ability to burn multiple files to a single writeable optical disc, but the way that those jobs execute (each one is executing its own instance of the engine, sbrun.exe) is such that the information for each job is essentially kept separate. Also by default they run in sequence (not in parallel).
On another note, wow, those are rather small volumes. You could just back them up to some hard disk and then use some other dvd burning software to burn BOTH images to the same DVD.
It's hard to imagine and provide for every possible use case under the sky. This is one of those cases. :|