Add the Actual Overtime work field to the timescaled portion of the Task Usage view. Right click in the timescaled portion, select Detail Styles from the shortcut menu, select Actual Overtime Work from the list. I would also show the Actual Work field.

Regarding your second question - did the work (16 hours) get done by one resource or was it only 8 hours of work? For the resource who did not work on the task, set the remaining work to zero. I would use the Task Form in the lower pane formatted to show Work.