Hi @Perez ,
Do you send email using Office365Outlook connector?
How do you get the predefined emails? Type them manually?
If you send email using Office365Outlook connector, and using two static email, you can try this solution.
1. Set OnCheck property of your checkbox to(save your two emails into a variable, remember to use ";" between these two address):
Set(varUser,"user1@onmicrosoft.com;user2@onmicrosoft.com")

2. Set OnUncheck property of your checkbox to:
Set(varUser,Blank())

3. For your send button, try this formula:
Office365Outlook.SendEmailV2(varUser,"title1","body1")
Best regards,
Allen