Assume you deposit 1-408-888-1212, and the GV will call you back at that number since that is the primary line.
GV can only make one call at a time, so if you want to make a new call on the primary line, it is not available.
However, you can do **2+number, to use your asterisk to call out.
Separately, you can have both SP's configured to the same GV, and set up a Trunk Group -then it will automatically hunt for the available line. More documentation to come later.