MPC: How can I specify a varying constraint that does not depend on time but on another variable?

Hi! I am using Simulink and the MPC toolbox. I need to specify a hard constraint for one of my output variables (y1). However this constraint is not constant, it varies and it is a linear funtion of another output (y2). How can I specify that? If I just connect the constraint to the mpc toolbox, whenever I go closer to that hard constraint the response oscillates endlessy and does not find any steady state solution.

Thanks a lot !

Sara