You may have 2 ways to merge data:

1. Purchase a 3rd party service to merge 2 QB Desktop company files into one QB Desktop file and you will convert the merged company file to QB Online yourself.

2. Purchase a third party service to merge 2 QB Desktop corporate files directly into one QB Online account.

Hello twbooks. QuickBooks makes it easy for you to migrate your data from QBDT to QBO. I encourage you to check out the following article that shows you how to: Migrate from QuickBooks Desktop to QuickBooks Online with Intuit's internal tool. Let me know if you have other questions as you go through the article. I'll be one message away in case you need anything else. 

Thanks for the info!

You're welcome.