Hi there, gilesbelfrage.

I'm here to help you import a CSV file for the second bank account in QuickBooks Online.

You'll be able to open the second account by clicking the drop-down arrow beside the bank account name. Please see the screenshot provided below:

Once done, you are now ready to import transactions from the CSV file. Let me guide you how: 

1. Hit the drop-down arrow beside the Link account.
2. Choose CSV file.
3. Tap Continue.
    

Lastly, I've got you this helpful article for ideas about how you can categorize your online banking transactions: Categorise and match online bank transactions in QuickBooks Online.
 

 Keep me updated in the comments if you have any other questions. I'll be here to lend a helping hand.