Im pretty sure 2024 will work like this:
If you buy two $400 Delta tickets on your Delta Reserve Amex, (one ticket for you and one for someone else), ignoring taxes and fees & assuming both the purchase and flight occurs in 2024:
You will get 400 MQD for the ticket plus 80 MQD for the DL Amex Reserve spend (40 MQD x2 tickets at earning rate of 1/10). So 480 MQD in total.
They will just get 400 MQD for their ticket, and that’s it.