Start by adding the client who will be signing for the event as a new Client, either from your Directory or create them as a new client in a project (Article), and then add the planner under the "Company Information" section (this way you'll be able to track how many projects you did with that planner across all of their contacts).
Then, when you're ready to send the quote, add the planner's email after you click 'Send to Client'.
Said another way, the quotes will still be sent to the planner (via 'Send to Client'), but the name of the contact on the project will be the planner's client.
You can learn more about adding/editing who is receiving the quote here.
Alternately, you can create the Contact with a customized email for the planner.
For example, if the planner's email is [email protected] and the contact's name is Colin Connor.
When you're creating the contact for the project, you'll add the contact's email as firstname.lastname@example.org.
This'll ensure the email will be sent to the planner, and you have a unique entry in your directory for that client.
Note: For this solution, we still strongly suggest adding the planner as the Company to ensure you can report on all activity for the planner, regardless of their end client.