Actually, I don't think it is possible to set up PH2 to completely "mirror" PH1 like an external line splitter widget would do, although you can come close -- such as having both lines ring simultaneously on an incoming call -- in software. But to have a true "extension phone", you'd either have to use an external splitter or, to employ Steve's approach, plug one phone into PH2 and a two-line-capable phone into PH1 and use the 2nd line on it, and then configure the OBi to use PH2 where it normally would use PH1.
Personally, on my OBi202, I have a 3-way device I got at Radio Shack some years ago. It consists of an RJ-14 plug and three jacks: Line 1, Line 2, and Line 1+2. Using this device in the OBi's Phone 1 jack plus the OBi's Phone 2, I can then connect a second phone to either of the OBi's Phone ports.
That said however, by it's very nature VoIP technology lends itself more to an office configuration where each phone is its own extension with the capability of being used independently to place and take calls, rather than the traditional home layout where every phone is an extension phone to a single line.