Depends on what exactly you are trying to achieve.
You can't share the same broadband connection on the same line
You could certainly have the two routers running independently. i.e main one is off the other one is on when you need to access the internet from the lounge. You would just need the same username settings in both routers and only one on a time. (I think with Sky they are hardcoded)
If its a wireless router you can often configure them as a wireless bridge where by it talks wirelessly to your main router and allows anything connected to its network connections to access the main network. This obviously depends on if you get a good enough wireless signal from one router to the other through the various walls in your house.
This would allow say a games console and media centre in the lounge to connect to the rest of the network and the internet. The quaility of signal though would determine the speed and whether it would be suitable for your needs.
Wired is always the best to go for speed and reliability but involves long cables or lots of drilling
