Hi, yes I am one of the maintainers of the official Delta Chat, ArcaneChat is the client I use myself, made to my taste, with my own opinionated decisions, also a lot of the features that are added to ArcaneChat are added to official DeltaChat eventually when the rest of the team is convinced it is a good idea or when we have the resources to add the feature to all platforms.
A recent example is: arcanechat.me server was the first server to completely enforce encryption without exceptions, there was some hesitation but now recently the official chatmail relay server deployment got this feature incorporated as well
So to answer your question about why there are two apps: because I can't force people to do whatever I want, so as it is good practice in open source, you fork