I know I've been posing many threads about transformers... I promise this is the last for a while
If a transformer has two individual 24VAC secondary windings, can they be connected as follows to create a 24-0-24 centre tapped supply and then rectified into a 33.6V dual power supply?
Apples and oranges. The circuit you drew provides two independent voltages a positive one and a negative one. The second one you propose would give one voltage but at twice the current. Same total wattage capability in both cases.
Yes, you can put them in series if you go with the two bridge rectifier method. You just put the 2nd bridge rectifiers + output to the gnd output of the first.
The only reason to do this is if you need really high current, or if you plan to use the two-positive-regulators trick to get regulated + and - rails using identical positive regulators (since some regulators don't have negative equivalents).