I'm not 100% clear on your explanation, but here is something to consider. If itemtype ONE has a property named bal_part that is an item reference to the PART itemtype and itemtype ONE also has a foreign property named bal_partname that is a foreign property showing the name property of the PART itemtype, then you cannot directly update the name property of the related part from an instance of the ONE itemtype. You'll have to write a method on add/update of itemtype ONE that updates the name property of the PART in the item reference of the ONE itemtype item.
You can control what properties are used to build the keyed_name. If you want the keyed_name of the part to be "item_number name" then the easiest thing to do is to set the Keyed Name Order value of the part's item_number property to 10 and the Keyed Name Order value of the part's name property to 20. The keyed name will be the concatenation of the item_number and name properties with a space between them.
