[RPG] Dynamic weapon enchantment in D&D 5e

It depends on the magic items in question.

It doesn't matter how many weapons or implements you equip/wield, you can only use 1 weapon or implement (the ki focus can be used through a weapon, but this counts as using the ki focus and not the weapon) on an attack.

Enhancement bonuses from multiple weapons or implements do not stack; only the enhancement bonus of whatever you use for the attack is added. The feat Dual Implement Spellcaster allows you to add a bonus equal to a 2nd implement's enhancement bonus to damage on arcane powers, but that's about the only way around this limitation.

Other properties may stack depending on how they're phrased. If the item's property says something along the lines of "when you attack with this weapon" then it can only be used when that's what you're attacking with. If the property has no such restriction then you get the benefit even when you attack with something else, though relatively few such properties are related to attacks.

Note that the Two Weapon Fighting feat has nothing to do with any of this. Any character can wield 2 one-handed weapons this way. That feat has nothing to do with magic items. Further, Two Weapon Fighting only adds its bonus to weapon attacks, which monks don't usually make.

The specific rules cite you mention is this one from the Adventurer's vault.

Many weapons have properties that provide a constant benefit. To gain the benefit of a weapon’s property, you must be wielding the weapon. Unless specified otherwise, a property affects only the weapon to which it’s attached. For example, a +2 cunning dagger, which bestows a –2 penalty to an enemy’s saving throws against your weapon powers, affects only powers that are delivered using that weapon. You couldn’t hold the weapon in your off-hand and gain the benefit of the property on powers delivered using a main weapon. (AV 56, Emphasis mine)

This is a funny little sentence stuck into the introduction to magic items in the Adventurer's Vault. The problem though is that it conflicts in two major ways with the rest of the 4e canon.

• It's not supported with a repetition in either major rules text (PHB/RC). From the magit item properties page in the RC:

  Unless otherwise noted, a magic item's property remains active for a creature only while it wears the item...or wields the item. (RC 280)

• It seems like the inverse is supported by actual weapon texts. If you read the weapon entries in the very text that has the limitation, only 4 weapons have a property that affects attacks but does not specify that it only applies to this weapon. The ones that don't: Subtle Weapon, Thundergod Weapon, Vanguard Weapon, Lucklender. Interestingly enough, errata has changed Subtle to specify that it's only "this weapon"* and the other 3 were left untouched.

This all comes down to what the word wield means in this context. I think you can find rules support for both holding and using for an attack. However, it seems like there are enough instances where it simply means holding it, that I would rule that the AV text is no longer valid. However, it's written, and that's sufficient for a RAW ruling that the magic item property only affects the weapon that is used for the attack.

This is something you'd have to take to your DM. You could make a compelling RAI argument that the items I listed above are clear exceptions simply for their lack of specifiers, the lack of support, and the fact that one of the 4 was errata'd and the rest were left alone.