Wow, that's strange.
It works on my Nvidia GeForce 5700 with multiple monitors (one digital, one analog, just like yours), no matter how I position the TF window -- all on one, spanning, etc.
I assume your drivers are up to date? If not, go grab the latest from nvidia.com.
It's always possible that it's a TF bug, but we really don't have much to do with multiple monitor support -- applications don't do anything special, or deal with it at all -- it's handled by the video drivers / card.
What's the resolution on the analog monitor? There have been some issues when the TF window is REALLY wide -- on some systems. However, I am pretty sure moving to "PRO" took care of these issues for people.