You can't use both lines on an analog phone with one OBi.
If you will be using just one handset, you can program the OBi such that SP1 and SP2 ring differently on incoming calls, and you have an easy way to select the outgoing account, e.g. 212-234-5678 would select SP1 and 1-212-234-5678 would select SP2. You could also set up your speed dials so some contacts go out on SP1 and others on SP2.
If you have a multi-handset two-line system, or several two-line phones, and your intent was e.g. while your wife is talking on SP1, you would like to be able to answer or call on SP2: You could connect two OBi devices, or use your OBi plus another ATA (Linksys, etc.) that you may have lying around.
If you have an IP phone with two or more line appearances, you can set up the OBi such that SP1 and SP2 appear on different line buttons.