2 pair being a long-term loser:

This is not good... 2 pair should be netting profit in the long term. You might be overplaying them (betting too hard with them, getting only made hands to call you).

Stacking off: You should really never make a decison in advance- better to look at where're you at currently. However in some siutations the pot is so large / the player's hand so good that even if the player knows there's a non-trivial chance he could be beat it still makes good sense to get it all in.