Hi,

Does any one know how to connect two units of LM338 in series in order to double the voltage?

TQ.

 

This is a text part of its datasheet

Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground.

Thus you only have to double the input voltage and take care of the above situation. Thus you only needs one Lm338