Ed Elliott shares some thoughts on merging dacpacs, including the all-important tl;dr:
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
That said, it’s not too long, so do read.