@Niko76 Hello, 

AV600 and AV200 should work together, but for the best performance, we still suggested using powerline KIT of the same specifications.

You could try to add extra powerline unit into existing powerline network by pair button referring to the instruction:
https://www.tp-link.com/support/faq/1081/

Note:  press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again)
When the Powerline Led or House Leds are on, the pairing process is done.