If you are working with a linearized model (order=1), the IRF to a joint shock is the sum of the individual IRFs. That way you can create any linear combination. If you want to do this more formally, you can use the simult_-function to generate any combination you like (even at higher order). See github.com/JohannesPfeifer/DSGE_mod/blob/master/RBC_news_shock_model/RBC_news_shock_model.mod for an example