How do I clone a mailbox on Exchange server to create a second copy with the same contents?

We have a situation here at work that 20 years ago used to be something you could resolve with a simple powershell command, but nobody in our IT department can figure out how to do it under Office 365.  

We have a long-standing shared exchange server mailbox (lets call it SERVICE) that is getting too big for one person to handle, so it needs to be split into two separate shared mailboxes (SERVICE-A and SERVICE-B) *both* of which need to retain the full contents and directory structure of the original SERVICE mailbox for historical lookup purposes before we divide their functions and they start running independently from each other. 

Ideally, what we need to do is create an identical clone of the existing SERVICE database on the server called SERVICE2, and then we can rename the two mailboxes to whatever we wish so they can go off on their separate ways. 

We cannot simply create a second mailbox and drag and drop the contents over because the original service mailbox has been around for decades and contains literally thousands of nested subfolders to keep the contents organized. One of the biggest headaches of M365, is that it absolutely craps its pants the moment you attempt to copy a folder that contains another subfolder and will immediately kick back a "cannot copy the folder because it may contain private items" error, even if you have full admin access to both mailboxes. (I'm sure Microsoft consider this a "feature" rather than an absolutely infuriating bug.) 

It would take us weeks if not months to go through and manually copy each sub-sub-sub folder over in reverse directory tree order by hand, so I need a better solution if one is available.

Cloning the original mailbox or overwriting one mailbox database over the top of another would be the most logical course of action, but sadly, nobody on our IT team knows how to do that. I'm not sure if that's simply because they don't have the correct documentation and powershell command, or if this is just yet another feature that worked perfectly fine for years under 2000 server, but for some reason, no longer functions under 365.