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TLDR; You probably don’t want to actually merge dacpac’s you probably want to deploy multiple and use /p:IncludeCompositeObjects=true.
If you do really want to merge dacpacs you can start with this sample: https://github.com/GoEddie/DacpacMerge
Why? I was reading the dacfx issues on the dacextensions sample’s github site: https://github.com/Microsoft/DACExtensions/issues and one of the issues https://github.com/Microsoft/DACExtensions/issues/23 asked about how to merge dacpac’s and I have been asked a few times how to do this and I thought there was a sample but I couldn’t find it so I knocked a quick one up.