Is the problem that 2 users want simultaneous access to either Adagio Payables or Adagio Ledger, and they are prevented from doing so? What is the message or error that they are receiving?

Do they have any multi-user Lanpaks installed? Check this by starting any Adagio program and check the Help/About screen. If you see '1' for the number of licensed users (instead of 0 of X), then no Lanpaks have been installed and this is the reason. If you see 'X of X' for the number of licensed users, then they are maxed out. The solution in these cases would be to purchase a (new) multi-user Lanpak.