I have studied Sequential Loop Closing or SLC in context of MIMO systems. Closing the loops sequentially while taking care of interactions make sense for a square MIMO systems. But does any theory exist for SIMO systems?
By SIMO systems I mean, underactuated system where you have more outputs than inputs, say the simplest case with $2$ outputs. In such a case combining the $2$ outputs to make a single output by some linear combination can be done but is restrictive. The question would be how can I analyze these restrictions?
In my particular case, I have two outputs and would like to design controller(s) that achieve a particular steady state response for the incoming input frequencies (i.e. required Bode Plot behavior). But for an underactuated system, I cannot do that independently using two controllers that are tuned independently. Is there a way to tune them together? Does a theory exit for that which limits the performance of these controller based upon which it can be commented if something is possible or not?
I can, in particular, give my exact control architecture but I think that would limit my understanding on a general level.