Designing flyback converter - multiple isolated outputs

Question

Can I treat 2 secondary coils with dual voltage output as one with double voltage for the purpose of calculations?

This seems to be OK for all formulas based on output power as technically the secondary IS a single coil with tap in the middle.

However for LT converters that use flyback in primary as feedback the formulas for feedback resistor(s) are based on Vout and turns ratio. How do I apply these?

UPDATE:

Allow me to clarify the question. Let's say I used all the right formulas and made myself a nice stable converter that puts out +50V at 100mA from isolated secondary coil and uses flyback voltage on primary as feedback.

Now, what happens if I tap the middle of secondary coil and convert this into dual voltage supply? Will I get a nice stable converter with dual +/-25V at 100mA outputs (minus drop on second diode, of course)? Without any changes in the feedback circuit or re-calculating the whole thing from scratch?

I am about 95% sure tapping output coil will not do anything to the functioning of converter, because the transferred power did not change. Except for that feedback resistor that according to LT8302 datasheet is calculated as:
Rfb = Rref * Nps * (Vout + Vf) / Vref
The Vout part of it is confusing, since now I don't have 50V, I have 2x25V.

Answer 1

Can I treat 2 secondary coils with dual voltage output as one with double voltage for the purpose of calculations?

With a flyback converter, energy is stored in the primary and released to the secondary at a rate dictated by the switching frequency. As most Flyback converters are DCM types, the output voltage produced depends entirely on the load resistor, the duty cycle, the input voltage, the frequency and the primary inductance: -

Picture source.

Regulation of the output voltage is performed by using feedback and this counters the effects of a varying load by applying greater or less duty cycle thus, it controls output voltage by increasing or decreasing the energy transferred per cycle.

However, for flyback converters with multiple outputs you have to be more careful. If one load suddenly takes less power, the other output voltage will rise and, in order to keep that voltage regulated, the energy throughput is reduced and the first load (that took less power) may become significantly reduced in voltage due to regulation feedback occuring on the 2nd load.

It's certainly worth modelling your load variations in something like micro-cap or LTSpice to see what happens.

However for LT converters that use flyback in primary as feedback the formulas for feedback resistor(s) are based on Vout and turns ratio. How do I apply these?

Assuming that method works fine for a single output (and it does but isn't quite as good as direct feedback) then you still have the same basic problem of what happens when loading imbalances can occur.