Nprinting actually requires a local qlikview desktop client to be installed on the same machine. Think of it as a driver for nprinting to access the infromation stored in the qlikview applications.
The QlikView Server can reside on the same or different server than NPRINTING server.
Either is totally normal.
Should you separate them ?
The type of load you get from NPRINTING often starts out as 'batch' (like publisher) where overnight a batch of reports is run and emailed. But if the users are using the ondemand features of nprinting the NPRINTING server will get load at any time of the day as users kick of paranmeterized reports whenever you want. Anyways just mentioning this so you get an idea of workload and when/where to start separating the components for load balancing etc...