welcomebot | Slack bot to welcome new members of a channel | Chat library

ActivityHandler implements IBot, so it can be thought of like a bot. Having multiple activity handlers would be like having multiple bots. Activity handlers are already designed to route different activity types to different code, so if routing is your concern then you only need one activity handler.

I presume your notifier is for proactive messaging. Rather than having a separate activity handler for it, what normally works is to have a separate endpoint, which is usually api/notify (as opposed to api/messages). You can still have a separate activity handler for that if you want, or not even use an activity handler for that case (like in the sample). Note that different channels may have special considerations for proactive messages, but that's outside the scope of your question.

Welcome messages are very easy with activity handlers. You can just use OnMembersAddedAsync in your one activity handler, and there's no need for a whole separate activity handler. Welcome messages are also channel-specific because they rely on conversation update activities, and not every channel has a well-defined way to know when a conversation starts before the user says anything. Here's a sample for if you're using Web Chat.

If you want multiple implementations of the same interface in your dependency injection then you'll need to identify them by the implementation rather than the interface, but keep in mind that you don't need to put them in dependency injection at all.