Tim Crall
I’ve created my second google account and added it as ‘Billing Account User’ – it shows up that way on the Account Management panel in Billing when I’m logged in as my admin account. However when I log in as the user account it doesn’t seem to be linked in any way, and when I create a project and then go to Billing it says "This project has no billing account" – if I click "Link a billing account", it says "You are not an administrator of any billing accounts"
Seems like maybe it was just a matter of taking some time; a few days later revisiting it, and my user account links right up the to billing account as expected.