Yes it is efficient to move data from one partition of TableA to an empty partition on TableB. Both Partitions must be on the same Filegroup AND the "range" from the Partition from Table A must match or "fit" into the targeted empty partition in TableB. There are some other restrictions that you may want to look at on BOL.
ALTER TABLE dbo.TableA SWITCH PARTITION 1 TO dbo.TableB PARTITION 1 ;