I've had my shopify store joestoes.co.uk for three years now. I'm in the UK bur half my customers are in the USA so I've just set up a second shopify store, us.joestoes.co.uk with prices in USD, easy shipping and Americanised spellings. I now want to publish my US store inventory to Google shopping but can't decide if I should create a new and seperate Google account for this. Any advice welcomed!
Same parent domain so setup under the same Merchant Account. Makes it easier to manage under one house. Personally I wouldn't use a sub-domain like the US one from a marketing perspective because it is confusing and conflicting with the UK TLD. For the AdWords account, unless you want to keep the billing entities separate for accounting purposes, I would put under the same account. It's simple to structure a campaign around the different stores with campaign names and regional targeting.