To add one or more assets to a TWS chart, select one or multiple data lines in a quote monitor or watchlist by holding the Shift key down and clicking the various financial instruments you wish to add. Right-click on one of the highlighted data lines and select Charts followed by New Chart. In the Add New Chart Window, you can select the existing chart you want to add the financial instrument(s) to and confirm by clicking OK.
As an alternative, you can click on the chart's Edit menu and select Secondary Series. Add the desired contract(s) and confirm by clicking Apply and OK.
You can find more information on charting multiple assets in the TWS Users' Guide.