If the displacement BC's are being applied to two different parts (or two instances of the same part) in the same step then it should work just as Drej suggested.
If the amplitudes are supposed to applied to the same part in different steps then that will work too. Just edit the BC in the appropriate step and change the amplitude.
If you want to apply two displacement amplitudes to the same part instance in the same step I am not sure you can, or why you would want to.