Mixed crystal of bis(μ2‐N,N‐di‐n‐butyldithiocarbamato‐S,S,S′)‐bis(N,N‐di‐n‐butyldithiocarbamato‐S,S′)‐di‐zinc(II) and bis(μ2‐N,N‐di‐n‐butyldithiopercarbamato‐S,O,O′)‐bis(N,N‐di‐n‐butyldithiocarbamato‐S,S′)‐di‐zinc(II)