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Depends upon how the jacks were wired. There is no such thing as a 1 line jack. Each line is in fact two wires, the basic RJ11 connector system provides 4 wires. So if the jacks were wired correctly, there should already be two lines available. Line 1 is the 'inner wire pair', and line 2 is the 'outer pair'.
Think of pin 1 and 4 as one line, and 2 and 3 are the other line(1st line).
If it was wired up correctly, then just connecting the other two wires for the other line at the service entrance would give you the second line on all jacks.
That is correct if you want to use 2 line phones, or single line phones on the first line.
If you want to use a one line telephone on the second line, you need a way for that phone to see the 2nd line. That is usually done with a splitter that takes RJ11 and makes it into two RJ11 jacks, one for each line, since a single line telephone expects to see the connections on pins 2 and 3 of the RJ11. The splitter simply takes pins 1 and 4 on the original RJ11 jack and connects them to 2 and 3 on the connector labeled L2. Splitters are cheap and available from sources like Radio-Shack, and other electronics outlets.
If you look on ebay, take a look at item 290282635855 (just cut the item number and paste it into the search box). That is a typical splitter used for this purpose.
There are two types of line splitters, one that simply convert the existing RJ11 into multiple jacks, and ones that actually split out the telephone lines. You want to split out the telephone line into line 1 and line 2 jacks to use a single line phone on an RJ11 with two phone lines.