Gmail – Moving one email from a conversation

This is by design, so it is not possible. There is no mechanism to merge conversations and the only way to split a conversation is to change the subject, even if it's only one character.

Forward one of the e-mails to yourself using the subject line from one of the other e-mails.

Unfortunately, that seems to be the only way, and the e-mails you forward won't be clearly indicated as having been sent by the real sender (nice header with their name in colour), but it will work to get everything in one place.

Given these two e-mails, like in your scenario:

E-mail 1:

• to Person 1 (manager, in your scenario)
• from You
• subject "Subject 1"
• body "Body 1" (question, in your scenario)

E-mail 2:

• to You
• from Person 2 (secretary, in your scenario)
• subject "Subject 2"
• body "Body 2" (answer, in your scenario)

...to get everything into one conversation, you send this e-mail:

E-mail 3 (a forward of Person 2's message, with the subject manually changed):

• to You
• from You
• subject "Fw: Subject 1" [need to override "Fw: Subject 2" with "Fw: Subject 1"]
• body "Body 2" (answer, in your scenario)

(I am saying "forward" instead of "reply" here in order to bring any attachments into the conversation too!)