Purpose of base-collector resistor in open-collector opto-isolated GPIO output
The voltage divider bias in this paralleled transistor circuit sets the threshold. Since you wanted to know the function of R66 &amp; R67.
The transistor, regardless if it is paralleled like above or a single transistor, must be biased so that when the base goes in positive voltage it saturates the transistor. So basically, it insures saturation of the transistor from the input signal, by setting a threshold bias.