Involving two things (6)
I believe the answer is:
binary
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">involving two things</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for binary that I've seen before include "System based on the digits 0 and 1" , "Using the digits 0 and 1" , "A sort of star" , "A system of two stars" , "Base-2 system" .)