Hi chauncy
This is a community driven forum, all community members can contribute to each other so you may not receive a reply from MYOB moderators. I'm sorry to see that no one has replied yet.
There is no shortcut way to merge the two company files, you'll need to manually move the data across then start recording everything in the single file from one point on. Whether to move the accounting data into the payroll file, or move payroll into the accounting file is up to your preference, and the amount of data involved. Please check the import and export fields page for all the data type that can be moved.
If you are moving payroll into the accounting file, the employee cards can be transferred, but the payroll categories will need to be set up manually. It's best to make the change over from the beginning of a payroll year, so the payroll data/reports of the same year aren't split into 2 files. You'll need to set up STP in the accounting file, make sure to choose you've previously used STP and provide the BMSID, please check the FAQ section of this Help page for details: Setting up single touch payroll reporting
Once you are ready to use the single online file only, you can then contact our support channels to have the other online file taken down. I hope this helps.