First, lets start with two transformers issue ..
If you can't find suitable multi-tap transformer (it can be CT or independent windings) then using several smaller transformer instead is not a problem ..
These voltage regulators will generate a lot of heat only when you draw a lot of current and the input-output voltage gap is wide ..
In any case, using 28Vdc to step down to 5Vdc is rather not a good idea ..
Much better option is working with much lower unregulated voltage, perhaps somewhere around 9Vdc ..
You can tie all 0V together but that may depend whether you use CT (centre-tap) configuration with 0V already defined or independent windings and full bridge rectifiers ..
This PS was originally used in an audio amplifier ..
The voltage that feeds U5 is quite high (32V or-something), so I think someone decided to use 12ohm resistor to drop the voltage down and possibly to delay the voltage, too .. ..
If you choose proper transformers the input voltages for voltage regulators will not be that high, and I wouldn't even think about resistors ..
Just make sure that you use decent heatsinks or maybe small fan(s) ..