Well, the problem with that ... is that the variable resistors would only control current.
But the correct operation of the LED's depends on the voltage.

If the total forward voltage of the LED's is less than VDD, both LED's will be ON no matter what the resistor values are. The only difference will be their brightness.

Given a specific VDD voltage, LED color (V forward) and foward current, the correct configuration can be found.

And to add 2 variable resistors to the flash demo, would increase the frames from ~50 to more than 500,000. I'm affraid that will take a few years to create, and you probably won't wait the 2 weeks to download it.
I suppose it could be done more efficiently.

But again, if you have Proteus, it's a "Piece of Cake".