They're the same controller, but some games support using them as two tiny controllers instead of one regular. You get a... Thing, that mounts them together for using it as a regular wireless controller when the system is docked.

You should not get a spare blue and red in my opinion. They're not amazingly comfortable controllers. For couch play, get a Switch pro controller, and use the red/blue as a two player controller when needed.

Using the red/blue as two separate controllers is mostly just a gimmick in my opinion. It might work OK for an 8 year old, but anyone in their mid teens and up are going to get serious hand cramps using them like that for more than 20 minutes.