Can the output of DC/DC converters connect in series?

Yes. As shown in below figure. It’s a positive and a negative output voltage  connecting through two converter’s output in series. It is recommended that the  output to connect with a reverse diode. The withstand voltage of the diode needs to be higher than the total output voltage of two converters.

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Input-Series Output-Series (ISOS) DC-DC converter circuit diagram.

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Input-Series Output-Series (ISOS) DC-DC converter circuit diagram.

Can the output of DC/DC converters connect in series?

Connecting DC-DC Converters in a Series to Obtain Various Output Voltages