We are based in London and are now opening an office in Austin, Texas (USA). Do we need to have a separate QB account or can we create a subsidiary company?

You'll have to sign up for a new QuickBooks Online account, Ken. 

That is because you will register your business in the US. States in the US have different sales tax rates. You won't be able to use the tax rates set up in your QuickBooks Online UK version.

I'll give you the links and articles to help you sign up and set up your new account:

1. Create a new company and add subscriptions
2. Understanding the Chart of Accounts
3. Set up and use automated sales tax
4. Connect bank and credit card accounts to QuickBooks Online
5. Set up payroll
6. Other supplemental articles to complete the set up is here.

Please tell us more on how we can help with setting up your account. We are here for you.