How can I enable two blocks by outputs of each other in Simmechanics?

In a multibody model constructed in Simmechanics, I'd like to enable only one of two blocks at one time. One of them is supposed to be disabled when the output of another is non-zero. How could I achieve this? Thanks in advance to anyone who can give any insight.

(These two blocks were constructed using Steve Miller's contact library, if it helps to understand my issue more clearly.)

[Lei Gao]

I've tried using if/else blocks with their output being the condition. However, an error occurs when I did so:

Input data dependency violation due to action subsystems. See Subsystem Examples in the Simulink library for valid and invalid examples of action subsystems.

Hope this information helps.