Multiple types of Damage

Are there combinations of weapons/items that cause multiple types of damage?

Yes...there are a few items, even at 14th Level that cause fire and thunder damage or fire and radiant damage for instance. Is that what you're asking?

Yes

Some weapons replace the damage done by a power with their damage type. Others are additive.

Aya uses a Crusader's weapon which deals half of it's damage as radiant, the other half is whatever the base power does.

Aya also uses Frozen Whetstones which add cold damage to all of her attacks. So she does Radiant/Cold on all attacks and can do Lightning, Psychic, or others in addition when using certain powers.

Note: it takes an action to use a whetstone, but there are items to make it a free action (like Aya's gloves).