Series isolated DC-DC converters fed by different PV sources
OK I understand. Try this: -



The two outputs are wired in series - this means current flow is determined by the weakest output without D3 or D4 being present. If one of the converters is powered down then its output impedance might rise to any value and I've made a guess at 1 kohm although realistically it could be between 10 ohms and several M ohms.

The diodes bypass the this output impedance and the small price to pay is one diode drop.