Concurrent execution in Simulink real-time

I have two model references - Slow model and Fast model, each running at its own rate for concurrent execution on the grt "generic real time" . However when I attempt to build the block I get the following error:

    Simulink cannot generate code for the signal at output port 1 of block 'Multirate/Fast' because the signal requires data transfer that generates lock-free code for a rate monotonically scheduled task.

I am not sure what to configure in simulink to overcome this error. I attempted to add rate transition from the Fast model to the Slow model but the error remains.

Any thoughts