Controlling 2 WS2812b strips with a NANO and 3 buttons

Other Hardware LEDs and Multiplexing
14

however if you press again strip 2 light 1 comes on.

Got it : please change :
  NumStrip1 = min(NumStrip1+1,NUM_LEDS);to  NumStrip1 = min(NumStrip1+1,NUM_LEDS-1);and the same in the other function.