I’ve used two together, for Input/Output or dual output. They were the Thunderbolt 2 version, on my Thinkpad P71 laptop via Thunderbolt 2/3 adapters from Startech.
I’ve used 2x USB3 Intensity Shuttle for simultaneous input and output from a 2017 MacBook Pro and an i7 PC, works fine.
I’ve also tried 2x output and that works also, but I did have some issues making sure the different Resolume outputs ended up on the right Shuttles after a sleep/reboot (though this was a while ago, might be fixed by a newer version of Resolume.)