Weapon cannot be prevented to break. The only way you can repair a weapon is by using staff called Hammerne but that staff only two avaible in the game and while secret merchant sells it, it is pretty damn rare. Weapon also lose its durability point if your unit retaliation don't miss, which mean if they attack but deal no damage, the weapon still lose one durability, including pair-bot or pair unit. This is a sole reason why units with access to mercenary class pretty important as pair bot since you can give them legendary weapon or max-forged brave/master weapons without worry.
Well, paired unit takes action each turn as one unit which mean pairing a healer even when they are not staff-locked anymore is a bad idea. Separate takes action, switching means that the pair unit cannot take action. The only purpose you do this is too give healer extra durability. Like, for example, I Have Lissa as a main healer for early game. But she is so damned squishy and can't retaliate, so I paired her with Kellam. When the turn nearly ends and I need some healing, I wouldn't worry too much if Lissa go to the front even if there are some enemies left, (providing they wouldn't one-shot or one-round) her because the extra stats should make her can take a hit or two.
And that attacking together is called Dual Attack, which chance based though there is a formula to it.