chungs, please do take a look at the other thread that amol12 has sent. It has lot of info.
@sachin_k5
It is no longer true that power outages will render cordless phones useless. Newer models (comes at the price) has features where in case of power outages the handset batry will power the base unit. Also some models support powering the base from directly the phone line power and the power adapter is only to charge the handset.
For your duplex house ideally look at a dual handset solution like the one below
KX-TG3712BX | Panasonic
I did not know your budget else could have suggested an imported DECT 6 seriies of Panasonic phones. Very good audio quality over distance. But the above model should also suffice you.