The free version of NoMachine, installed on the computer you are connecting to, allows one connection. To connect to the remote computer, you can install any of the following on your local devices: NoMachine, Enterprise Client, the NoMachine app for mobile. If you try to run two connections from different client devices to the same RPi where you have installed the NoMachine free edition, it won’t be possible. However, since you have two separate RPIs, each one with their own NoMachine installation, running one connection on each of them at the same time from the same connecting client will be possible. What is counted (on the server side) are the permitted connections running on the machine you are connecting to.