How can I restrict a user (with PPSK) to connect a SSID with only two devices? I observed that, after created a PPSK user, the same generated password can be used to grant acess for, e.g., a Iphone and a Ipad. Sorry, I am a novice in PPSK (and Aerohive AP too).
If you look at the SSID configuration page a few lines below where you configured the use of PPSK, you will see a check box called Set the maximum number of clients per private PSK.
Check that box and indicate the max number of concurrent devices that can use the key (2 in your example.) Only two devices can use the key at one time, with this field set, but they can be any two devices.
If you want to restrict the use of PPSKs to specific devices, you can also check Automatically bind a private PSK to a MAC address. When a device uses a PPSK, the PPSK will be bound to the device going forward. However, this is a little more complicated to set up, as you have to configure an AP to be a PPSK server with a static IP address. The PPSK server is the device that remembers the binding of PPSK and MAC.