.....................................
  u  :  +--------+ y1    u2  +--------+  :  y
 ------>|  sys1  |---------->|  sys2  |------->
     :  +--------+           +--------+  :
     :................sys.................

 sys = series (sys1, sys2)
 

     .....................................
     :                   v2  +--------+  :
     :            ---------->|        |  :  y
     :  +--------+ y1    u2  |  sys2  |------->
  u  :  |        |---------->|        |  :
 ------>|  sys1  |       z1  +--------+  :
     :  |        |---------->            :
     :  +--------+                       :
     :................sys.................

 outputs1 = [1]
 inputs2 = [2]
 sys = series (sys1, sys2, outputs1, inputs2)