Gruuping not working

Volumio 3 Issues?

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Hello !
I have problem with grouping wich is not working.
I have two devces Rpi 3b+ 2x
One is main (server) activaded with premium subscription, and second with free sub. On first under sources multiroom is enabled, on second is not becouse, it is under free subscription and this is slave device.
When I open output on MAIN volumio, and select gruuping, the second slave device is not visible.
What is problem, shoul I have premium on slave or not?
How I can log with premium on SLAVE? When i type same credentials on slave, Im logged but it switch me to free subcription, so I cant enable multiroom.
In manuals is written, that one premium is enough for six devices controlling.
Any suggestions?

both devices must be logged in with a premium subscription

just log-in with the same email and password on both (the one with premium subscription)