Removed
Yes, I would definitely add two separate nodes. In addition you can tag the floor with level= and draw the building from Bing. If there is just one name I would add it to the building. If there are different names I would add them to the appropriate nodes.
Similar to shopping centres, e.g. BigC Khon Kaen or Central Plaza Khon Kaen
Removed
As far as I know applications usually process only one primary tag per node or way, e.g. http://www.openstreetmap.org/browse/way/31821942.