1. How to enable Split Conversations? 

1. Click on the Settings icon on the lower-left hand corner. 

2. Scroll down to the section labelled Settings & Characteristics, and click on the relevant module (ex: FB Messenger). 

3. Click on the channel you want to make the change in. 

4. Click on the subject Split Conversations. 

5. Click on Allow and in the row underneath, select how long after you would like the conversation(s) to be split.

2. A customer reached out to us; at the end of the conversation, the agent marked it as "completed". Afterward, the customer responded again in the completed conversation, and the conversation was transferred back to the agent to be handled from the beginning, even though split conversations had been activated. Why?

You need to verify within the relevant channel settings that Split Conversations is toggled to On. A short time afterwards, Split Conversations should appear in the relevant channel settings as On.