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You can use a One Temperature sensor and One DDC controller, which can accommodate all the output points of the two FCU's. You will deal with the two units as if they are One AHU with Two 3-speed fans, Two 2-stage heaters, and two cooling valves.
Although, it will be an expensive solution.
From HVAC point of view, you need to know why you need to run them both with one Thermostat?
The FCU is a zone HVAC device. so it serves only a small area. If you are going to let them both work this mean that you have a big area. In this case, I think it's better to control each one of them with its thermostat.
i think it is not possible, each fcu should have separate thermostat to controll it.
The two FCU shoud be run alternatively in timer mode
you should get a thermostat with two stage setting to control 2 FCU's primary and secondary)
