does both verbs have to be imperative because I am confused
no, in the combination of two verbs (like in your example "przestań narzekać") the first is imperative, the second infinitive.
but the imperative gets different forms for different persons:
Sing.
1. -
2. Przestań narzekać (Stop complaining)
3. Niech (on/ona) przestan
ie narzekać (Let him/her stop complaining)
Plural
1. Przestań
my narzekać (Let's stop complaining)
2. Przestań
cie narzekać (Stop complaining)
3. Niech (oni/one) przestan
ą narzekać (Let them stop complaining)
3rd person isn't technically imperative, but I forgot what it is, but it has the same function.