Open map who you want to coppy .
Click Ctrl+P or go to Map -> Properies.
And look for CLient Version :
In 2 map you must have the same Client Version to opening 2 maps at the same time.
If you don't have the same Client Version enter a little change and save it -> do it on 2 maps , and you have the same Version.
Next open 2 maps on the same time .
And you can Use Shift to check area who you want coppy , use ctrl+c and ctrl+v.
Dont forget about it :
you must have it if you want add selection.
If you want coppy all floor you must change it in Edit -> Selection -> Lower floors.