Another thing you could do - and I'll leave it to one of the more expert people to describe how it's done because I haven't done it yet - is setup a route so that if you dial one of the GV numbers, it automatically routes the call out through the other number.
My wife and I have the same setup you describe. My GV is SP1, hers is SP2, though I set SP2 as the default outbound calling provider since she makes more calls than I do and I don't mind prefixing mine with **1. But if I want to call her, I will have to do that as well or else it'll dial her voicemail; at some point I'll set up the calling routes, I'm still in the "playing" phase so I'm not too worried about it yet :>