If what you have is two blocks (or subsystems) both inside the same model, then you could use https://www.mathworks.com/help/simulink/slref/ratetransition.html rate transition block .
I am not familiar enough with Simulink to know all of the conditions under which blocks can end up running at different rates. I know that when you use a Simulink buffer block or a conversion between pixel streams and frames such as convertToFrames that the other side of the block is considered to run at a different rate.
