Welcome to the forum. Yes that's no problem, when you set up your new box just login with your usual username and password, you can have up to four devices registered to your account and watch NOW TV on up to two at the same time.
I would also recommend enabling your Payment PIN in your account, this will prevent accidental or unauthorised payments in the future. If you have a NOW TV Box it's easy for people to accidentally purchase a pass. To turn your Payment PIN on, go to your My Account > Payment PIN page
It's pretty simple all you need to do is when you setup the second stick and when prompted use the same identical email address and password that you used on the first stick on your account.
Once the stick is setup and activated watch something with your nowtv pass already on your account and the stick will be automatically added to your device list.
And you can now have six devices on an account, though you are still limited to two simultaneous streams.