I can see the relevance of having the two companies merge as one, @duncanholden. Let me explain this further to you.
Combining two or more companies is more like incorporating all transactions from one another. Although, there isn't a way to merge two companies into one subscription since one subscription is equivalent to one company in QuickBooks Online. What we can do is, import your data and transfer them manually to your current account.
Here's how:
- Sign in to your Company.
- Go to the Gear icon.
- Pick the Import data option.
- Choose the data you need to incorporate with your current company.
I'm also adding this page for your guide and additional insights into importing data in Quickbooks Online. Commonly asked question is also available here: Common questions about importing data to QuickBooks Online.
Do you need to know how to switch views within your company? Additional information can be found here: Switch between Business and Accountant view in QuickBooks Online.
Ping me on this thread if you have another query. I'll be glad to help.