I was surprised but relieved to find that I wasn't the only one to have thought about this and that a vital feature had been already included by Sentinel in 7109's design.
In order to use one engine only, the steam to the other needs to be isolated. However this is not sufficient as air has to be allowed in instead to prevent the isolated engine from drawing a vacuum when the valve timing dictates that steam should be entering its cylinders.
This is how it works.
|White insulated main steam feed pipe|
|'Y' splitter highlighted bottom right|
(Click the picture to enlarge it)
|Vital feature highlighted to the left|