DHT dht (DHTPIN, DHTTYPE);
Creates an instance of the DHT object named dht
DHT dht1 (DHTPIN1, DHTTYPE);
DHT dht2 (DHTPIN2, DHTTYPE);
would create 2 instances of the DHT object named dht1 and dht2. Note that you will need to #define both of the pin numbers before you can do this.
Later in the code use each of the two objects as you would use a single one. You could also create a two element array of DHT objects but I suggest that you do not do this initially
As to the motor drivers, simply add more #defines for the pin numbers and use them but please give them sensible names that indicate which motor driver they relate to.