With the microcontrollers the outputs can operate as source or sink.
So a circuit like this is possible if and only if
the sum of the forward voltages of both LEDs exceeds the voltage of the power supply. Only blue and white LEDs do the trick. When you get an LED check the data sheet for the UF value.
So you really have three possible states:
If you don't have blue or white LEDs you might take 2 LEDs of any
other color to replace one of the blue or white ones.
Just make sure that the sum of the forward voltages of all your LEDs is not less than the supply voltage.
If you still run of ports, check for