You only need one OBiTALK account, and you can add as many OBi hardware devices as you wish to it, via your online portal page.
So, you would have the work OBi and the home OBi both on the account. Doing so doesn't affect how they behave with regard to Google Voice. You need to set up Google Voice for each box, individually.
Like I said, each box can have up to four Google Voice accounts assigned to it.
If you want to configure the 9389 number, it's entirely up to you: you can configure it only on the home OBi, or you can configure it on both the work and the home OBi. You need to assign the 9389 number to a separate Google Voice account, and configure it on your OBi(s) as you wish.
Example:
Work OBi:
SP1 = Google Voice account with 9583
Home OBi:
SP1 = Google Voice account with 9389
or you can also provision SP2 on either or both of those boxes with both GV accounts, one on SP1 and the other on SP2. The portal will let you pick which number to use as the default for outbound calls, and which number should cause the telephone to ring on inbound calls.