I cant find a plugin for sync'ing permissions across servers, 
But couldnt we just make a hardlink to the file so both servers pull from the same file? (local servers)

I'm guessing the issue would be that one sever would add a permission/group and the other server wouldnt see that until it got rebooted and read the file again?

or does oxide hold the file open so no other oxides can write/read to/from it?

im using linux but in windows(as most ppl use it)

mklink /H c:\RustServer1\serverfiles\oxide\data\oxide.groups.data c:\RustServer2\serverfiles\oxide\data\oxide.groups.data

Any files under oxide/data are only loaded on first load, and are held in memory until saved to or restarted. You won't be able to copy those files over for them to be read on-the-fly. You can use the same file between both, but you'll likely run into write conflicts if they both try to write to them at the same time and potentially corrupt files.