Here's how the voltage regulation works, and forgive me if I'm telling you things you already know:
Every LED has its own current-voltage relationship. When you put a certain level of current through an LED, it will produce a corresponding "voltage drop". For example, if you run 1,400mA through your 3618, the voltage drop across it will be about 100V. If you run the same 1,400mA through a CLU048-1212, it'll only drop 35V across the COB. If you were to increase your current to 2,100mA, your 3618 voltage drop will jump to ~105V and the 1212 would jump to ~37V.
Constant current drivers output however much voltage is required (within their rated range) in order to produce their set current through the whole circuit. If the driver tried to push 2,100mA through a pair of 3618s, it might require a voltage of 105.2 for the first COB plus another 105.3 for the second COB (voltage can differ even between COBs of the same model), so it will adjust its output voltage to 210.5 in order to push 2,100mA through the whole circuit.
This is the reverse of using a constant VOLTAGE driver with an LED. In that case, you're choosing the voltage rather than the current, and whatever voltage you decided to run the LED at will produce a corresponding current draw. Using the same example, if you were to set a constant voltage driver to 105V and hook it up to a 3618, you can expect to see it draw around 2,100mA, and the current would be allowed to vary in this instance, while the voltage would be the thing that's held constant.
