Decision vs Optimization version for Problems of two Parameters
Having more than one input doesn't change anything:

Consider an optimization problem $MinQ$: 


  Input: $x, k$
  Output: find $y$ satisfying $Q(x,k,y)$ which minimizes $f(x,k,y)$: 
  $$g(x,k) = \min_{y : Q(x,k,y)} f(x,k,y)$$


The corresponding decision problem $Q$ is:


  Input: $x, k, l$
  Query: is there a $y$ satisfying $Q(x,k,l)$ such that $f(x,k,y) \leq l$?


To solve the decision problem,
we can solve the optimization problem,
then check if the output is less than $l$.
I.e. there is a very easy reduction from $Q$ to $MinQ$:


  input: $x,k,l$
  1. $m = MinQ(x,k)$
  2. return $m \leq k$