Having one key that opens the deadbolt and the doorknob making it easier to lock and unlock your door.
Carrying one key instead of multiple reduces the bulk on your keychain and the chance of losing your keys. It’s easier for family members to enter with a single key especially in emergencies.
If the key is lost or stolen it would give someone access both locks making it easier for them to enter your home.
Using Different Keys for Each
Having different keys for each lock can provide an additional layer of security, for example you can give out the key to the doorknob service providers or temporary guests and limit their access when you want to by using both locks.
You can also control who has access to different parts of your home more effectively using this system. To have 2 separate keys can be sucks if you’re lose one key or if you need to give extra keys for family members.
In an emergency having to use multiple keys can cause delays or confusion for occupants or responders trying to enter quickly.