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
myMod: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 current workaround (version 1.5) I found to make it easier (although this seems weird).