So on the primary machine you have the set the Share Permissions for the folder with the QB files so that they can be shared across your network?  (Use a WIRED network if at all possible.  QBs Data often has issues over a typical office WiFi.)

Then on the Primary machine you've gone into QBs and set it to Multi User Mode.

Then on the Second machine you've checked to see that you can reach the QBs folder on the Primary machine.  And finally, opened QBs (Multi User Mode) and tried to open the QB Company file...

Let us know which part doesn't work (in the above order) and we'll see what we can do.