Many laptops come with two graphics adaptors, usually the Intel integrated + another GPU like geforce. This is so it uses the low power intel graphics for windows and the higher power graphics for games. But this is usually done from the factory, its not practical to retro fit usually.
Some HIGH end laptops will have two Geforce cards to run SLI for more performance. Again this is usually done from the factory, its not practical to retro fit usually.
Laptops do not have graphics cards, they have graphical chips soldered onto the motherboard, usually as part of the CPU. Sometimes the chip will be separate from the graphics card, but not usually. If this is true, you still cannot replace the chip unless you are really good at micro-soldering.
Google MXM graphics card and you will see there have been lots of different laptop graphics cards.
MXM cards are relatively uncommon and a pain in the ***.
Doesn't make it not exist? They are used quite a lot actually, even some new laptops are using them.
If we go down this train of thought, we are off topic. I also very highly doubt the asker of this question even understands the concept of mobile gpus, as he used the term graphics cards and not GPU or MXM cards.