This is my understanding ....
When creating the liveCD, the curent xorg.conf is excluded.
That would not mean the exclusion of any backups to that file I suppose
During boot of a liveCD, a lot of hardware detection takes place .... including the graphics system.
So if the graphics card is present on a PC it *should* be detected. That applies to two graphics cards.
Both should be found.
So your SIS & Nvidia appear to be detected.
The only means, as far as I am aware, of preventing detection of the SIS card is to disable it.
For onboard graphics that is achieved through the BIOS.
If the BIOS for some quirky reason does not allow that, then you are stuck with the cards presence being known by the booting system.
Now, let me propose a theory ......
there are two cards in your system when booting; both are found and recognised; how does the booting system know which one to use?
In a normal install, it is determined by the xorg-conf file, I believe.
But in the case of a live boot, the xorg-conf is generated during the boot, and its content is determined by the hardware detection. (true?)
The on-board detected card is given priority (or detected first). (true?)
There would come a time when the connection of a monitor to the graphics socket would be detected. That output should then be the primary output being used. (true?)
It might appear then that the monitor-connection detection is happening after
the log in.
How about that for a theory?
Well what do you make of that?! Where is this Unichrome coming from?!
The card is detected during boot, because it is not disabled.
All the above, is of course, just musings.
It is a theory created to try to explain the facts