> This always happens, since they are driven form two crystals.

Hm, the two devices aren't always driven from two crystals.

For example, on OS X -- I have a built-in input device for my master output (which is the clock reference) and then a separate built-in input device for my microphone. They are the same system device but they are separate devices to Mixxx. They share the same crystal. This change adds a 1.5x latency delay to my microphone unnecessarily. The current master works perfectly.. they operate in lock step with no overflows or underflows.