Juggling two separate accounts seems to unnecessarily complicate things for the users.
But for those that want and use separate accounts, why not just configuring the chat account as an additional account in the classic email client, to have access to the chats and be able to move and copy messages?
Concerning the double notifications, in many cases the problem found a perfect solution after disabling the notifications in the classic email client, and only have DC emit notifications. Quickly, those close contacts that should trigger alerts tend to also use DC. And classic email becomes again a check-when-time-is-right task.
But granted this simple “only chats trigger notifications” is not for everyone, so either
- server side email filtering (as in your linked blog post), or
- DC emitting notifications for classic emails
are still very valuable options, allowing for separate chat and classic email notifications without double notifications.
Juggling two accounts seems complicating things in particular, because it adds the new types of problems you mentioned.
Compare it with having a metric that distinguishes between (short and compact) chat-style versus document-style emails. (Something like 98% correct, 1% silent start-chat? prompts, and 1% wrongly opened tentative-chats that can be closed again with a simple touch of an “X”. But my guess is the errors could be about zero in reality.)
The metric would, for example, also still allow to (silently) show classic emails only as icons in the chats, available for reference (and interactions) and providing a chronological overview.