Does a Series Voltage Reference referencing more than one electrical block has to be buffered?
With voltage references loading can become a concern, especially if accuracy is desired. Loading the voltage reference will sacrifice accuracy by a very small amount. 



This probably doesn't apply to you since the load described above in the OP is about (a high impedance load like an opamp is negligible input current, I'll consider the sine input to be zero, and the micro ADC will also probably be less than 10uA, so I'll throw it out too) $$ \frac{3.3V}{20k+20k} = 0.082mA$$ so the error will be $$ \frac{20ppm}{mA}*0.082mA = 1.65ppm$$ not anything to worry about, unless the application was using a high accuracy ADC, just something to double check if you have a low error budget. 

Make sure the capacitance loading is correct on the reference (it needs 0.1 to 10 uF) or the regulation will be poor. 

So what the loading was an issue, what could you do? Put an op-amp buffer in between the voltage reference and the load.