However, I want to have the option to switch between the 2 numbers in terms of what shows up on the caller ID. Can anyone tell me how hard that would be to do?
With two Google Voice accounts configured on an OBi, one will normally be your PrimaryLine. By default, calls will be placed using your PrimaryLine account. To use the alternate account, you would prefix the dialed number with **1 (if your PrimaryLine is SP2) or **2 (if your PrimaryLine is SP1).