The length of the cable does affect the image. 20 feet should not be too bad. It would be better to have one 20 foot cable rather than connecting the 2 10 footers. A computer interface unit would correct your problem but it is expensive. It would increase the signal strength and correct for the length of cable.
For the last year I had my laptop connected to my LCD tv via a VGA cable. In my new room arrangement my computer is further away from my Tv so I went and bought 2 10' VGA extensions, Dynex brand. It works, however, the picture is not as clear. You can almost see a duplicate image behind the main one. Does anyone know if this is caused by the extreme length the signal has to travel or because of the extensions or perhaps the extensions are just plain faulty. Any insight would be much appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic