actually
 

the dragon slayer sword loses morale when NOT fighting dragons.. when you fight against them, the item morale shoots through the roof! otherwise i think you have to suppress the gremlins every so often to keep the item's morale at 50/50

Huh? Why not? If I can un-equip the item to prevent its morale from suffering when I fight certain monsters, then why can't I carry a secondary weapon in my inventory to equip when I do that?

i think
 

i think he just means you can't have two equipped at the same time.. you can carry as many as you like in your inventory and equip/unequip them whenever you please.

the warrior can equip 2 weapons though =)

No, you can have only one weapon in hand at a time, but you still can easily store other weapons in your inventory

So, I'm getting confused by the responses here. The original question was: If you have an artifact that loses morale when fighting particular foes, is it effective and logical to carry a second weapon that you can switch to before fighting those foes? Is anyone saying that won't work?

I guess it could be a problem if your primary weapon adds leadership, and you switch to a weapon that reduces your leadership so you can't control your forces. Anything else?

Short answer.
Items in the inventory are treated as if they do not exist.

Anyway, anyone know if high morale do anything?

it all depends on you) I used two swords in the beginning of the game (normal sword and a rapier with hit bonus when fighting against undead), in the late stages I was carrying only one weapon, 'cause i was constantly forgetting about the need for switching from one type to another) what is more, none of the artifacts is vital for playing, so i did not panic when my arcmage's staff morale was about 30 and dealt with its gremlins only when i wanted to))

Are they more important at the highest difficulty? Or not really?

More, but imo still are not worth paying special attention to them. If i found useful artifact - it's good; if i didn't - i didn't really care (this was on hard difficulty. didn't play impossible). Usually i took only one thing that had a morale with me - it was that archmage's staff. In all other cases i preferred to wear things with smaller bonuses but without morale