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In the two guest VMs, the serial port will be used to transfer data to the other VM. Using e.g. LapLink, InterLink. The usage will not be limited for file transfers, but also be used for remote debugging, so the serial port is required and communication by other means are not applicable.

Set up a communications pipe (VM settings|serial). Configure one side as the master, the other side as the slave. Be sure to start the master side first.

Thank you. It works. :)