Hi,
Can you Private Message me the phone line numbers in question and the full install address so I can d some investigating?
REIN is not a problem anyone wants to have since it is normally down to local electrical interference outside on BT / Openreach's control. Openreach only have a handful of REIN engineers and their time is in great demand so the wait can be horrific.
As an immedate plan of action I would suggest putting a seperate modem/router on the bad line and raising it as a fault with your broadband provider since they will need to try and book a REIN engineer. The second thing to do is review when the broadband drops and see if you can see a pattern. The cause will be local so follow the phone cable routing of the bad line noting how this differs from the good one. Then check any electrical device within 5 meters. It could be a faulty power supply, extension cable or the heater kicking in.
Simon.