If you order the 2nd line from BT and put it on the same a/c # as the 1st line, your existing Call Plan covers both. So you will only need to pay line rental on 2nd line.
The BT cable to the premises carrying the 1st line usually comes with 2 line pairs. So the 2nd line can be taken off it immediately.
If you order the 2nd line from some other provider, it is usually BT Openreach who installs it unless you go for a Full LLU provider like TalkTalk.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 19 Meg Tweaked / 16 Meg Untweaked LLU BB