It is possible to "fake" it.
You can run several instances of calibre simultaneously on the same computer if you use VMs, Virtual Machines, using for instance VirtualBox. Then you can install and run calibre in each VM. As far as calibre is concerned it is the same as running calibre on dfifferent computers. But for you as a user it is
almost like you are able to run calibre and have more than one library open at the same time. Almost, because you wont be able to drag and drop between the libraries. But you can have more than one library on screen at the same time.
https://www.virtualbox.org/
Plenty of RAM and several processor cores helps. Otherwise it may feel too slow.