your previous post suggested both gain and loss shares would be sold. however, just selling the gain shares can create an issue.
there is a methodology that you probably have previously established for which shares are sold. generally, you selected either FIFO - the first shares bought are the first shares sold (the usual default method with brokers if LIFO is not selected) or LIFO - the last shares bought are the first shares sold.
so if you sell the gain shares, depending on the methodology:
if FIFO you would use the first shares bought even if in a different account
if LIFO you would use the last shares bought even if in a different account.
The IRS doesn't care that they are in different accounts.
so the 1099-B you get may be incorrect.
the broker will most likely report what they have as the tax basis for the shares sold. you may need to adjust this when doing your 1040.
