So if i understand well, even two SC downloading at maximum can not turn it on?
If you have two SM's... that have optimal seperation across the AP to trigger MU-MIMO (groupable STA's), and the SM's are NOT being limited by a QoS/MIR throttle... and they're running at/near full modulation... then yes... they will trigger MU-MIMO and you should see speeds across the AP double, and you'll see the green 'MU-MIMO' downlink graph indicate this. You can test this by using the Wireless Link Test on the AP... and running tests between two 'groupable STA's'.