In general more LEDs per output don't work very well.
You can try to put 2 (or maybe 3) LEDs in parallel. the brightness will be reduced because the same amount of current will flow through 2 LEDs instead of 1, so you will have to increase it in the PRo Shift configuration screen.
Because the 2 LEDs are never identical, it is possible 1 LED is brigher than the other. you have to try