Input Manager Question

Yes. Just go to Edit-> Project Settings -> Input .... then in the inspector panel, make a new input axes , name it "Vertical" ... then on the positive button, type 'mouse 0' and on the alternate positive button type 'mouse 1' ... or whatever you're wanting to use.

Note : in case you're unsure how to make a new input in there, where it says 'size' change that number to add one more input axes, then go down and name it whatever you want the input to be named and set it up how you want it

Edit : I might have misunderstood your question. If you're wanting it to not do anything unless both buttons are being held down at the same time, you could do something like setting one of them up as the positive button, but then in your code do something like this ->

function Update(){
  if(Input.GetMouseButton(0) && Input.GetAxis("Vertical")){
     //do stuff ;
  }
}

then, if the mouse button is held AND the vertical button is being pressed also, it will do stuff.