In the UK?
You could save money by having one line removed, as you're possibly paying rental, probably to BT, on both lines.
You can buy multi-handset phones anywhere - Tesco for instance.
Only one of the handsets needs to be connected to the telephone port. The rest just plug into any mains supply around the house. I have 2 downstairs and 1 upstairs.
I believe you can have more than one telephone number using the same line. BT use packet switching technology to enable line sharing with no noticeabe drop in audio quality. It will affect broadband speed though.