Yes you can add either one or both at the same time, HP2551 or HP3501 displays. You can have as many as you want.
The way it works is that each sensor basically acts like a miniature radio station and each broadcasts a one way signal. You can then have as many receivers of this signal as you want which are all the different consoles; GW1000, HP2551 and HP3501 and even some others. Think of it as a radio station broadcasting and that radio station can have as many people in cars and in their homes tuned in to that frequency to listen to the broadcast, but at a much smaller broadcast range. The sensors have no idea how many consoles are picking up their signal....the sensors just broadcast their signal with their data. It is then up to the consoles to be compatible with the sensors that you get.
