In Flex 6400, QSK works for the same antenna only for version 2.49 (2.5 did not check), and starting from 3.019 only for different ones. When you turn on one antenna for reception and transmission, QSK is immediately turned off. What is the reason?
Problem solved! Tim explained that starting from version 2.5 there is no possibility to work with QSK on one antenna if the ATU is installed and enabled. In my case, Flex 6400 + ATU. That was the problem. When you turn off the ATU, QSK runs on a single antenna. The question is removed!