With the Find Positions in All Accounts tool, you can quickly see your exposure to a particular security across all accounts or sell all shares of a position. To open the tool, select Find Position in All Accounts from the Accounts menu, or from the Action menu of any appropriate tool, including Filters, Watch List, and Positions. The symbol in focus will launch the tool, seeded with all of your positions in the security, across all accounts. To sell all shares of a position across all of your accounts, select the Close All button. This will seed the Multi-Trade tool with the order so you can close the position.
You can choose to close a position in one or more (as opposed to all) accounts at one time by using Shift + click or Ctrl + click. Once you're finished selecting accounts, right-click to choose the desired trade action. Your orders are then added to the Multi-Trade tool, where you can edit or place the trades as you would normally.