Switching Regulators in Series
If the switching regulators have common ground between the input and output, you cannot put them in series.

If the input and output grounds are isolated, you can put them series.

What is limiting your 42V input to a single regulator?
If it is the input capacitor, replace it.
If the switching device, MOSFET mostly, cannot handle 42V across its source and drain, replace it with a one with higher rating.

If there is no MOSFET but an IC doing all the stuff, connect a MOSFET (with proper rating) to the switching output of the IC and move the inductor and capacitor (filter) to the output of the MOSFET. Make sure that the pulses' peaks coming from the IC are within the specification of the MOSFET. Too low peaks will not turn on the MOSFET, too high pulses will damage the gate of the MOSFET.