Updating only one of two maps in QGIS Print-Composer without updating other map

On the map item go to 'Item Properties > Layers'. There are two check boxes:

"Lock Layers" = prevent any layers being added or removed from the map window

"Lock styles for layers" = prevents styles changing on layers visible in the map window.

Checking both of these for a particular map item stop any changes happening to that map so you freely make changes to a different map window.