To add a bit to SteveInWA's answer, once you have both accounts set up, incoming calls on both numbers will automatically ring to your phone. If you want, you can choose different ring patterns for them. Under the Voice Services SP1 and SP2, you'll find a setting called X_RingProfile.
If you want to call out on the second number (SP2), you can designate it on individual calls a couple of ways. One is to precede the number you're dialing with **2. Another is to include that in the speed dial.
If you want to exclusively call out on SP2 for some period, you can change a setting on Physical Interfaces/Phone Port 1. The setting is called Primary Line.
After you submit your changes, you might want to reboot.