Use Texture Atlas addon to unwrap objects as one into new UV map, set up the UV islands scale as desired and use that map instead.
After activating the addon in the User Preferences, in the Properties editor, Render tab find Texture Atlas rollout and click '+' button to add new one. Options to make automated or manual unwrap will be available. For more control choose Start Manual Unwrap in the bottom of the rollout (.
What it does is the addon temporarily "joins" selected objects (they will be added into group and atlas will be created for them) and adds UV map with the same name per each one. With manual unwrap mode active, you edit objects as if they were one, assign seams as needed and unwrap. If you have UV maps already created select the meshes and in UV / Image Editor find UVs > Seams from islands.
Once finished, exit Manual Unwrap mode by clicking Finish Manual Unwrap button. Objects will again become "unjoined" ones.
Before unwrapping using the addon:
After unwrapping is done objects use UV map named the same for all of them and scale of the islands is set up properly:
The scale of the individual islands will be already equalized to correspond between all the adjacent objects.
Texture Atlas addon's page (the addon is already included in Blender).
baking to multiple objects at once with multiple materials
Arrange UV maps of multiple objects without joining them