Is it possible to generate the pop with only one H9? It's possible that this is an issue with interaction between the two H9's, in which case you might need to play with the timing and routing of your MIDI changes.
As my colleague mentioned above, it is very difficult for us to remotely diagnose issues with complex setups… there are many things that can be going on that we can't possibly see or know about. The best bet is to find a qualified technician to help you out, or to patiently break down the problem and setup yourself, to find the least complex possible setup that makes the problem happen. Unless you can do that, it's going to be a Sisyphean task for us to help you.
