They just connected them together and didn't worry about it. The sync voltage is derived from the main HV supply via a high value resistor. If you connect together two flashes with differing sync voltages, they will settle at the higher of the two. When triggered, both trigger transformers will then produce an ionising pulse to start the flash tubes.
Originally Posted by PeteZ8
If both flashes have sync voltages from about 60v to 200v then they should work fine together. If one is low voltage (5v or 12v) and one is high, I wouldn't connect them together.
I looked into the Cactus triggers. They use an opto triac which is rated at 400 volts. Probably the same component is used in modern cameras which are rated up to 250v but many paranoid people will not connect anything over 6v to.