I want 2 tiny businesses (A and B) to share one COA. Is it advisable to use two main accounts for A and B separately, then create sub-folders for each business?

It's recommended to use one QuickBooks Online (QBO) subscription per company file, Market. I'll provide detailed guidance below to help you maintain accurate records.

You cannot merge two company files into a single chart of accounts. Hence, each company file requires a separate subscription. However, QBO allows you to create multiple companies under one account, which you can access using the same sign-in information. Having all your companies in one place enables faster switching and more efficient management.

If you are considering creating subfolders and subaccounts, it's advisable to consult a professional or reach out to our QuickBooks Live Expert Assisted to maintain precise financial records.

You can also utilize the location tracking feature, which allows you to categorize and group transactions by location for streamlined deposits.

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@marketviewconsulting  I mean, you could.

The real issue is whether you will want a balance sheet or any serious financial statements for these companies later on, or to accept most forms of electronic payment from customers.

To the former, QB has enough problems extricating a balance sheet by class, let along something like this.

To the latter, 1099-Ks are meant to be tied to one TIN. That does not work well when you have two companies in a trenchcoat.

If you do plan on wanting those reports or receiving those payments, it's not a good idea.

If you do not, well. It's not like it would be that big of a deal, but also, you'd be overpaying. If your accounting needs are that small, you'd do just as well with Excel.

@marketviewconsulting 

Another option, use an old QB Desktop with a non subscription license.

@marketviewconsulting 

It depends on what you mean by "tiny" and what type of entity these businesses are (sole proprietorships, S-Corps, etc.).  Depending on which version of QBO you're using, you can use classes to separate income and expense transactions and create income statements for each business.  Otherwise, if you have a simple COA, separately identifiable income and expense accounts should be adequate IMO.

A shortcoming of QBO is that you can only post A/P and A/R transactions to one default account, respectively.  Therefore, your receivables and payables will be combined for both businesses. 

Also, QB will close combined net income at year-end to a single equity account that should be allocated to separate equity accounts unless both businesses are sole proprietorships.

Similar to what FforA mentioned, 1099s could be an issue, but I think only if you're issuing them and have separate EINs for each business.  Even then, you can issue 1099s through a different system.