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The Flex Viewer only accepts one web map, so "no" you cannot use two web map directly.
Some alternatives:
* Create a new web map 🙂
* Convert the web maps to operational layers. There's a handy utility, Web Map Converter, for that at: http://resources.arcgis.com/en/help/flex-viewer/concepts/index.html#/Helper_applications/01m30000002...
* Write custom flex viewer code to convert both web maps on the fly.