The following code is an example of 2 scales for the same building

-- Register the primary root part
myMod:register({
	DataType = "BUILDING_PART",
	Id = "MY_PRIMARY_ROOT_PART",
	Mover = { DataType = "BUILDING_PART_MOVER_INSTANCE" },
	ConstructorData = {
  	    DataType = "BUILDING_CONSTRUCTOR_SCALER",
	    CoreObjectPrefab = "MY_PRIMARY_ROOT_PREFAB",
	    EndPart = "MY_SECONDARY_ROOT_PART",
	    FillerList = {
			"MY_PRIMARY_TILING3_PART",
			"MY_PRIMARY_TILING2_PART"
                },
		BasementFillerList = {
			"MY_PRIMARY_TILING1_PART"
         	},
		MinimumScale = 1,
		BasementScale = 1
	}
})
 
-- Register the secondary root part
StoneGateV6:register({
 	DataType = "BUILDING_PART",
	Id = "MY_SECONDARY_ROOT_PART",
	Mover = { DataType = "BUILDING_PART_MOVER_BRIDGE" },
	ConstructorData = {
		DataType = "BUILDING_CONSTRUCTOR_SCALER",
		CoreObjectPrefab = "MY_SECONDARY_ROOT_PREFAB",
	 	EndPart = "MY_SECONDARY_ROOT_TOP_PART",
		FillerList = {
			"MY_SECONDARY_ROOT_TILING3_PART",
			"MY_SECONDARY_ROOT_TILING2_PART"
                },
		MinimumScale = 0,
		BasementScale = 0
	}
})

Note:

1) “MY_PRIMARY” is the basis scale, “MY_SECONDARY is the upper scale

2) 2 green arrows appear. There are a little bit tricky to set. Mover = { DataType = “BUILDING_PART_MOVER_BRIDGE” } is the workaround I found to get it easier (although this seems weird).