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.

Energies, Free Full-Text

Dc Converter - an overview

power supply - AC/DC Converters in Series with the Same AC Source - Electrical Engineering Stack Exchange

How to know if the 2 output capacitors are connected in parallel or series : r/ElectricalEngineering

The Ground Rules to Select and Build a Modular DC/DC Power System-IV: Load consideration

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

Accurate DC-DC Converter Minimizes Wasted Power

2 dc to dc converter in series? - General Electronics - Arduino Forum

Using DC-DC Converters in Parallel - Technical Articles

DC-to-DC Converters: Types, Uses, In-Circuit Design, and Firmware

Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter