It is almost certainly possible. There are a number of potential problem areas:
1) Making your BIOS provide access to both display interfaces. If I'm correctly reading the files you provided, your BIOS has disabled the built-in display adapter and allowed only use of the AGP card. Am I correct in assuming you selected the AGP card as the "primary" display (for the BIOS to use during power up) and it is the only one Linux was using when you prepared the files you posted?
You should save copies of the /etc/X11/xorg.conf and /var/log/Xorg.0.log files as they are with GUI Linux working on just the external card. Those files will be helpful in figuring out what to put in xorg.conf to use both cards.
Maybe there is a BIOS setting to enable both display interfaces (but it would still use just one for the power up display). If you can't find that, it is likely easier to make the BIOS enable only the internal. The nVidia driver for the AGP card can be made to enable it even if the BIOS didn't. That is probably not true for the internal interface.
2) Getting xorg.conf right to support both display interfaces, and I expect you also want to tweak it such that booting Linux while one or both monitors is off won't confuse it (so you can later turn the monitor on and have it work). The best info for all such tweaks to xorg.conf is the single monitor versions automatically created by a Linux installer or other auto config methods when you are using just one monitor on one card.
More difficult tweaks are best supported by changing things in xorg.conf and seeing the differences that creates in Xorg.0.log
3) Maybe you also want advanced features, such as extending one desktop over two monitors. Multiple monitors in Linux defaults to one desktop per monitor, so you can move your mouse between monitors and work on each monitor (open new applications on your choice of monitor), but you can't move a window from one monitor to the other nor span anything across the boundary. Most people prefer single desktop. But that takes extra setup effort.