IB has added a new feature named "Custom Securities" to the latest version of the TWS.
Not only does Custom Securities support "a/b", but it gives you the flexibility of incorporating any number of securities into a free-form equation.
To access Custom Securities, right-click on an empty real-time quote row, and choose the "Custom Securities" item (you may need to expand the drop-down menu using the down-arrow at the bottom of the menu).