You can't forward a google voice number to another google voice number in the google voice settings, so it's definitely not that (it tells you "google voice cannot forward to another google voice number").
As far as the second, I did not set that up in the obihai. is it set that way automatically?
Doing some further investigation, I think I know what it is. I have both gv lines also forwarding to my cell phone, and my cell phone is set up to forward on no answer to gv line 1. so i'm guessing when there's no answer on gv line 2 when it's ringing the cell, it's forwarding to gv 1 voicemail.