Question about TP-Link Re450

When is the TP-Link RE450 Duo recommended? We live in a three-storey house. We have our Sunrise router on the ground floor, which brings the WLAN signal up to the first floor with a reasonably stable connection. Could I then plug in the TP-Link Re450 on the 1st floor so that I can boost the signal up to the 2nd floor? I read somewhere that the TP-Link must be plugged in close to the router (with a clear view of the router and no walls or the like in between). Would this mean that I need two of these TP-Link Re450s? One on the ground floor right next to the router and the second on the first floor so that I can get the signal up to the second floor? The next question(s) would be, why do I have to pay just under CHF 40.00 for the single TP-Link Re450 and just under CHF 140.00 for the TP-Link Re450 Duo Pack? Couldn't I just buy two single repeaters for CHF 80.00? or can the single repeaters not be connected together like the Duo Pack? Many thanks for your efforts in answering this question. Greetings from the IT greenhorn himself ;)