Same Stock in Two Different Brokerage Accounts

[Al Smith]

Hello!
Can anyone tell me how to do this in gnucash?
I have investments with a brokerage house (Brokers R US) split between two
different accounts (account #1234 and account #6789). There is some overlap
in the equities contained in these accounts so that both accounts have 100
shares of the stock XYZ. I would like the account tree in gnucash to look
like this;

Assets
     Investments
          Brokers R Us
               Account #1234
                    Stock
                         XYZ
               Account #6789
                    Stock
                          XYZ

However, when I try to add XYZ to the second account, an error message pops
up saying "That commodity already exists.", and I cannot proceed.
I've tried making another account under Investments called Brokers R You
then adding the sub-accounts Account #6789, Stock, and XYZ but I get the
same error message. I'd really like to have both 1234 and 6789 in the same
gnucash file, in order to simplify recording transfers. I can't simply
combine the two accounts into one at Brokers R Us, since income into each
one is treated differently for tax purposes. Can someone suggest an
approach?
Thanks.