Trying to split the current output of an LED driver
The LED driver regulates current, so it adjusts the voltage until the current is 20mA.

If you put two LEDs in series, the LED driver will still regulate to 20mA, but the voltage required for that will be the sum of the LED forward voltages.

The downside is that you need a high enough supply voltage, and any channel that has only a single LED will need to drop the voltage in the driver. If it's a buck converter, fine, but a linear regulator will heat up quite a bit.