Why does this optocoupler's datasheet recommend splitting a current limiting resistor in two?
What @Andy said is basically correct but  the reason for the split R's is more complex.

It has to do with the insulation  transient impedance ratios for the  lightning simulation test specification.

The device capacitance in the off state = 77pF @ 1MHz @ 0Vdc  and the Partial Discharge (PD) rating is in uC.

Highest Allowable Overvoltage (Transient Overvoltage)

V IOTM 6000 8000 V peak

In this case the impedance is not the ESR of the diode or the resistance of the Series resistors but the relative capacitance divider ratio.  

The diode capacitance being larger than typical resistor capacitance (including dust, Pollution degree 2) (over a wide range of case types) permits the device to ensure it can handle over 7x the DC insulation value of  1140 V peak for isolation.

8kV is a standard impulse test spec and thus they found 1.5 anode/cathode R ratio to work best for immunity to 1us rise time pulses.

I won't attempt a mathematical answer but it requires model each part as a // RC equiv cct each in series.







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