Two master sockets on the same line can adversely affect the quality of the telephone service. The amount af degredation depends on the other equipment ( telephones, modem, hubs, extension bells, etc ) is in use and how they are wired / plugged in to the system.
The other concern is that the automatic routine line tests will almost certainly detect there are two masters on a line and this will create a flag on the files for that line that the Network Termination may not be standard. This isn't a problem until you have a fault on the line and ask for it to be repaired. When it is suspected that the Network Termination has been "modified" then the help desk protocol towards the customer may be different.
Two master sockets on one line will probably do little for ADSL performance either.
Providing the sockets are master sockets, not NTE5s, with a split front plate, and correctly connected together with 3 or 4 wires, a master socket can usually be converted into a secondary socket by removing the capacitor and resistor from the circuit board.
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