The reason why you got an error was you were trying to define currentState a second time.
byte currentState = digitalRead(SWITCH1);
The above was where currentState was defined.
Since it was defined, you can then use the variable again and again, just don't have byte in the front.
And YES you could have done this too:
byte current1State = digitalRead(SWITCH1);
. . .
byte current2State = digitalRead(SWITCH2);