I have a Volt and a Buick on my Onstar account and they exist fine together. There is an option in both online and on the iphone app to change vehicles when looking at stats or wanting to remote start etc.
You can also now get the individual iphone apps for each vehicle type so you could have one for the Volt and one for the GMC (as long as GMC has a separate app).
I didn't have to do anything. It all magically happened with the onstar people. I am in Canada though so I think it is slightly different than the US version which maybe why mine works ok.
It's important to use the same email address when setting up the OnStar accounts.
We first leased a 2013 Volt and I was the primary on the paperwork (with my email address). We then bought a second Volt (2014) that my wife drives but again I was the primary on the paperwork and used my email address. I could see both the 2013 and 2014 in the phone app and the OnStar website, no problem. Finally, when the lease ended on the 2013, we bought a replacement Volt but my wife thought this time it would be cool if she was the primary on the paperwork (and that included her email address). The dealer set up OnStar for us before delivery and he gave them my wife's email address. Now we find that to check charging status, etc., on the 2 cars we have to constantly log out and log in with the other car because the login ID's are the email addresses. We would have done things differently had we realized the inconvenience this would cause.
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