Can I add a second new business to one QuickBooks subscription running two separate businesses

It's great to see you here today, @geoff8. 

You can add a second business under your QuickBooks Online (QBO) account. However, each company you create will be a separate paid subscription. Please take note that you can access them all with the same login credentials by performing the steps below. 

1. Go to https://quickbooks.intuit.com/choose-country/.
2. Select Asia Pacific, then click Australia. 
3. Choose a product (Simple Start, Essentials or Plus).
4. Click Free 30-day Trial or Buy Now.
5. Since you already have an account with Intuit, click Sign in or Subscribe here beside Already have a QuickBooks Online account?. 
6. Enter your Email Address (User Id), First name, Last name, and Password.
7. Click Add new company.

The screenshot below shows you the first three steps. For detailed instructions, go through this article: Create or Subscribe To Another Company. 

Now that you have two companies, you can quickly switch between them to manage everything more efficiently. Just go to the Settings (Gear) icon at the upper right. Then, select Switch Company under Profile as seen in the screenshot below. For more details, check out this article: How Do I Switch Companies?. 

I'm just a comment away if there's anything else you need. Have a great day, @geoff8.