You can plug 2 power strips into one outlet as long as you don't overload the outlet.
As long as your total amperage usage of the appliances, electronics etc that are plugged in do not exceed the amperage of the outlet which is either 15 amps to 20 amps then you're safe to plug 2 power strips into the one outlet.
I have 2 power strips plugged into one outlet although I don't have all the things turned on at the same time that are plugged into those power strips.
If I did try turning everything on then it might be too much amperage for the one outlet.
You can also plug one power strip into another power strip to increase the amount of outlets but just don't overload the power strips by using too much electricity at one time.
As long as you are safe when using the electronics or things that are plugged into the power strips then they are safe to plug one into another.
Also just don't plug a space heater into a power strip as they can overload and melt.
Space heaters should always be plugged directly into the power outlet without an extension cord.
A good power strip will also have a breaker that will trip when the power strip is being overloaded.
It's usually a push button on the power strip that you push back in to get the power going again.