The problem came in with them changing it after it was copy protected... There was talk of it, but it caused too much hassles with the implimentation (since the copy protection interferes a lot with the game handling) that it was abandoned again... i am sure in the next game those options will be there, since they already tried that with BD.
@ wallace
You do know that you can open more than 1 inventory at a time...
to do this, select 1 character (not both), click the inventory button (not the shortcut, since that will cause the next one to close rather than to open another), and MOVE the inventory to another place on the screen...
then select the OTHER character and again click the inventory button (not the shortcut).... and you will see both inventories... you can still change character in each inventories, so you CAN have both screens on the Hero (i.e.) and work like that...