Tomshardware

we will need more information.
which external graphics card, laptop, and what you will be using them for would be helpfull.

In all likelihood, no. I remember a couple of times when this was suggested, but external graphics cards would require an extremely high bandwidth connection to the laptop, and most laptops do not support it.

Yes it can, GOOGLE ATi XGP and you will have most of the answers to your question.

It's the extension of the original LASSO solution, and it now works both on external and internal monitors, but the selection of supported laptops is limited.

The ATi XGP seems to require a special port that is added to the notebook itself, also the port seems to be not standard and not available generally.

The old Asus XG Station was an intended product to do this, but somehow it never really went retail.

The XG station was a single PCIe1.0 lane from the ExpressCard slot, which is pointless for graphics. Other single lane solutions like the ViDock and other smaller brands have been out for a while (with PCIe housed solutions out before the XGStation was first expected to launch), but are equally useless and do not work in tandem with the internal card as the OP asked.

The XGP is 8 PCIe2.0 lanes, and as I said, while limited, is the only worthwhile solution, just needs to be seen in more laptops. If there were a single standard that would likely help, but as the XGP is based on the external PCIe standard it's at least a start in the right direction if a very small one.