AWS SES Reply To Previous Email

With more complicated mailing schemes, filling the "References" and "In-Reply-To" headers of the proceeding message with the "Message-ID" generated by SES for the first message can nudge email clients to keep both messages in the same thread.

For instance, my use case was Alice referring a client to Bob. First, I wanted to send a message from Alice to Bob with the client's info (not seen by the client); then a message to the client, from Alice, CC'ing Bob, introducing the client and Bob.

I think it will not be possible, as a reply is nothing more than a new message that email clients decide to group by some criteria (eg: title and sender). You can try to send a new email with "RE:" before the original title and test if your most common clientes (corporate email, gmail, outlook, etc.) group as you expect.