As far as the voltage source is concerned the Zeners are connected in series, not back-to-back ..
Reverse one of them and then you can remove the resistor from the equation ..
Hi,
I would say, a current limiter (your resistor) is (practically) overall needed!
You must see it on as a complex of 3 serial components, & not only the 2 x diodes, as a two pole element...
K.
Depends on the purpose of voltage limiting. My comment was related to the shown circuit, a real voltage source hasn't zero impedance of course. But if it's able to source a lot of current, you should add a series resistor between voltage source and limiting diodes.