I can see how this feature can be beneficial for your business, especially in billing multiple customers.
Adding two or more customers to a single invoice is currently unavailable in QuickBooks Online. You can, however, duplicate the invoice and make the necessary changes and use it to bill the other customer.
Like this:
Go to Bookkeeping, choose Transactions, then All sales.
Find the invoice or estimate you want to copy.
In the Action column, click the dropdown ▼, then hit Copy.
Edit the invoice or estimate as needed, then select Save.
Also, QuickBooks Online is constantly evolving, and new features are regularly added based on user feedback. Thus, I recommend sending feedback to our Product Development team so they may consider adding it in the next future update.
Here's how:
Go to the Gear icon at the top.
Click on Feedback.
Enter your comments or product suggestions. Then hit Next to submit feedback.