While bug hunting on my own synth I found an issue where garbage audio would be produced in one instance if another instance was playing audio while Logic Pro X was in play mode. The problem seems not be an issue in ableton (neither two vsts, two AUs, or mixed would produce the problem). I looked at my instructions and nothing was public that shouldn't be as far as I could tell so I tested the demo plugin to make sure that it was my instructions that were causing the problem and it seemed two work. I compared my instructions to those of the demo plugin and nothing obvious stood out. Then I realized that my version of the demo plugin was compiled in juce 3 and that problem might be in the juce 4 api so tried recompiling the demo plugin in juce 4 and when I tested it sure enough the issue came up.
sorry if the explantion is too long but I just wanted to be clear.
Edit my version of Logic is 10.2.0