How to reduce the effect of one of the manipulated variables in an MPC controller with a MISO model as the internal model?

I am using a MISO (2 inputs) model as the inernal plant model in an MPC controller and I want one of the manipulated variables to have maximum effect on the output.I tried to adjust the weights of the manipulated variables but still could not get a desired effect.How to fix this issue?I am using MPC toolbox from MathWorks for designing the MPC controller.