How can I create an output of a system that is a linear combination of two existing outputs?

If I have a system G (either state-space [ss], transfer function [tf], or frequency response data model [frd]) that has one input and two outputs that have been labelled as such:

 G.InputName = {'i1'};
 G.OuputName = {'o1','o2'};

How can I change the model to have one input and one output that is a linear combination of o1 and o2, for example o=o1-02 ?