Of course you can install a 2nd adaptor. I've got a game that needs Pixel Shaders and it's junk so I need to do the same.
But...
First warning...if you have no experience in opening the back of your computer... be very careful. Asking for help would be wise.
But onto the gee can I do this?
1. where's the video plug in for your monitor right now. If it's next to the keyboard and mouse connections its probably "integrated". In english that means it's on the motherboard of the computer.
2. So next to the back where your keyboard/mouse are you'll probably see a bunch of silver slots. Those are expanansion slots that you can use to add stuff (like graphic cards) to your computer.
3. The easiest thing you can do would be:
a. take the make/model number of your computer to a local computer shop. Confirm that you can run a "PCI" video card.
You probably can, and you'll need some advice as to what specific card to buy.
reminder;;;;be careful on what you buy. The graphic card MUST be compatible with your game or it's junk.
I'll assume you're running Windows XP. If you're not, it's much more difficult.
So:
1. buy the right PCI graphic card
2. know how to open the back of your computer. If you're not sure get help.
3. When the card is installed (that's physically put in the back of your computer in an expansion slot) and close up everything. (with the computer OFF of course)
You will still have some software/display stuff to do with Windows. That can be dealt with separately, but needless to say you would have some fussing to do with the 2nd display adapter.