I'm not entirely clear what the problem is but, if it helps, you can tell what type of item you've got in one inventory and create an identical item in the other inventory with code such as this:
using System;
using System.Collections.Generic;
public class Item
{
/*some variables */
public Item(){}
}
public class myItem : Item
{
public decimal Amount;
public myItem Clone()
{
// shallow clone
return (myItem)MemberwiseClone();
}

public myItem(){/*some variables*/}
}
public class Inventory : List<Item>
{
public Inventory() {}
}
class Test
{
static void Main()
{
Inventory inventoryA = new Inventory();
Inventory inventoryB = new Inventory();
myItem mi = new myItem();
mi.Amount = 100m;
inventoryA.Add(mi);
Item it = new Item();
inventoryA.Add(it);
foreach(Item item in inventoryA)
{
if (item is myItem)
{
myItem oldItem = (myItem)item;
oldItem.Amount -= 3m;
myItem newItem = oldItem.Clone();
inventoryB.Add(newItem);
}
}
Console.WriteLine(((myItem)inventoryA[0]).Amount); // 97
Console.WriteLine(((myItem)inventoryB[0]).Amount); // 97
Console.WriteLine(inventoryA[0] == inventoryB[0]); // False
Console.ReadKey();
}
}

Yeah I guess I wasn't being clear enough, so I'm sorry for that. I should have said that my DropItem method was inside of my Item class, and a call to it is made from my Inventory class.
So inside the DropItem found in the Item class, what I have is:
for (int i = 0; i < mobile.inventory.Count; i++)
{
if (mobile.inventory.ElementAt(i).GetType() == this.GetType())
{
int amountLeft = inventory1.ElementAt(i).amount - i_Amount;
int amountToDrop = inventory1.ElementAt(i).amount - amountLeft;
inventory1.ElementAt(i).amount = amountLeft;
Item item = new Item();
//Cast item as type of this.
item.amount = amountToDrop;
inventory2.AddItem(item);
}
}

for (int i = 0; i <inventory1.Count; i++)
{
if (inventory1.ElementAt(i).GetType() == this.GetType())
{
int amountLeft = inventory1.ElementAt(i).amount - i_Amount;
int amountToDrop = inventory1.ElementAt(i).amount - amountLeft;
inventory1.ElementAt(i).amount = amountLeft;
Item item = new Item();
//Cast item as type of this.
item.amount = amountToDrop;
inventory2.AddItem(item);
}
}

Why don't you make the DropItem method in the Item class 'virtual' and then override it in each of the derived item classes?
That way, when you call DropItem() on an Item reference, the run-time type of Item will determine which method gets called and you won't need to worry about casting to the correct type in order to access the appropriate members.

That's not such a bad idea. I might drop the "cast it" idea and go with that. Thank you for your help!