I don't think they are using two green screens for Weather Center Live because it would take to much time to walk from the HD studio to the other green screen by the Digital Media Center.
I just think the settings to key in the maps somehow changes and is not correct.
Not only is the key not full with white colors gray but the dark colors on the OCM's have a bit of a higher contrast on them.
The same thing happened last night. The maps were fine for 7pm but then at 8:10pm when the weather updates started the map keying on the green screen was not 100% in it stayed that way on the 10pm and 1am WCL.
HD Studio Green Screen with correct map keying in.
Green Screen with map key not 100%