Or if you have a pod or pump then you can use only the rapid acting and the software will drip it slowly to cover your basal need over the 24h.
Trying to use just the fast acting with self-injections doesn’t work because you go high overnight while you’re sleeping, and even during the day you would have to be injecting every 3 hours to keep up. (E.g. when your pump controller dies on vacation and all you have on hand is rapid-acting)
Source: I have two kids with T1D.