Rx 580 4gb- single card 26mh 2 cards 20 mh why?

Hi i recently joined mining community… i have 4 rx580s 4gb verison… so i have tested each card when i recived it sepretly and all get around 26mh when i put 2 cards in the system and run them on same overclocks i get 20 mh i am wondering why is that… (i only have 2 slots in my motherboard still waiting for risers) oh and also what overclocks do you guys recommend i am curently running Core 1150 DPM 1 V 790 MEM 2100 MDPM 2… also i was thinking of bios flashing i am wondering if it is worth it for a litle improvment… i am happy with 26mh on a single card… but as i have said i get 20mh when there are 2 cards in the system…
PS. sorry for bad english;)

I suppose that you are using lolminer in zombie mode. Right?