I don't think you can setup a "Master" and "Slave" unit. Why? Because the controller doesn't read it as that, I don't think.
When setting the two MP's up in Fusion, I believe it gives you the option to run "Anti-sync" from the other, which is exactly what I did.
If anything, I'd say the Apex is the "Master" and the two MP40's are the "Slave" since its telling each one what to do and when, especially in anti-sync mode.