We're still working on features that will make it easy for users to merge content from two Haiku Decks. For now, here are a couple of options:
1) Use the copy decks feature. If you have content in one deck that you want to use as a starting point for a new deck, you can always copy that deck and add/remove slides from the copy. More on copying decks here.
2) Export to .ppt and merge using another app. If you export multiple Haiku Decks in .ppt format, you can open the exported files in a traditional presentation tool (PowerPoint, Keynote, etc.) and merge using those apps. Export is a feature of Haiku Deck Pro. More on how to export from Haiku Deck here. A support article on merging decks within PowerPoint is here.