Seems this situation is ideally suited for feed-forward control: measure the concentration in stream 2 upstream of the mixer and manipulate stream 1 flow rate.
Vivek P. Utgikar, Ph.D., P.E.
Professor, Department of Chemical and Biological Engineering
University of Idaho
875 Perimeter Drive, MS 1021
Moscow ID 83844-1021
Phone: (208) 885-6970; Fax: (208) 885-7462