Switchin High DC voltage
It may be the case, that using multiple DC/DC converters is the most space and cost economic variant anyway. At a 10 kV voltage level, it's somewhat naive to place a DC/DC converter first and ask then "how to switch the output?".
You didn't mention dynamic requirements. The situation can be different, if a particular switching behaviour is needed in your application.
Regarding available switching devices, the said spark gaps are unsuitable (apart from a too low voltage rating), because they perform "switch-on" only. But reed relays with 10 kV rating are available, e.g. from Meder, for fast switching, cascaded MOSFET can be used.