That's a Help page and is not contractual. There is no condition in their T&Cs restricting LRS on more than 1 line.
I phoned BT's Customer Option Team, who deal with such things, yesterday and they said I could have LRS on both my lines, but when I said go ahead and order LRS on one of my lines, they replied the system is refusing it because I'm already in a contract.
So, as an existing customer, if you want LRS, the thing to remember is that when you are taking out or renewing a contract, renewable or not, for your Calling Plan is to order LRS at the same time.
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