[unboggling]

There are some workarounds even if you don't use DropBox or other FileHosting/Syncing service.

One way is to make a copy of the library, then make only that copy available to the other instance of calibre on the other computer. And remember to use it only as Read-Only, any changes made won't cross over to the original. That was the way I was doing it to have 2 different libraries open on side by side computers.