Hello Mollycoddle,
While at my previous address, some years ago I did have a second BT line installed. I was at that time a Talktalk customer for BB and telephone. I do recall having a pretty poor BB performance, which eventually led to a BT engineer calling. If I remember correctly, He ended up switching the connections up the local pole, which gave a more stable connection, but didn't really improve the BB much. At the time I was only using one of the lines, I think.
Don't know if that's any help
TTFN
toulouse