I still dont see the reason for two 'maps'. If I understand this correctly, you are just talking about two groups of layers.
Group them - Now and Proposed and publish as such.
In the Now group you have your now fire station layers (plural)
In the Proposed Group you have your new fire station layers (plural)
The Swipe tool will work with each group as the swipe tool looks at the base level of each item in the TOC (LayerList)