That's quite good for an unregulated output. Is your 140V load not tolerant of that much change?
I've seen more like 40% regulation, on naively wound transformers. So that's not too bad.
The solution is to:
- Sausage-effect some of the error into the 12V rail (assuming they are common ground). Use a divider resistor from each supply down to the TL431 error amp (if this supply is isolated in the usual way), so that both are jointly regulated with some weighing factor determined by the ratio of resistors.
- Reduce the leakage between secondaries. Design the transformer with transmission line transformer methodology. Get the impedance down by coupling windings more closely together and using wider, flatter wire (or multifilar in parallel, or foil). If the resulting increase in isolation capacitance is troublesome, consider using a larger core (more Ae) with less gap to use less winding length.
- Split it into two independent converters. Annoying, but easier to design the transformer, and no worries about cross regulation with isolated outputs. (Note that, if the outputs are isolated, it's not enough to put an error amp on each channel -- that handles the excess voltage on each channel, but does nothing about the shortfall of voltage.)
Tim