How to make a donut biome ?

[Cestal]

I'm learning how to create biomes. But I can't create the shape I want to have:

I always get two biomes side by side

Here the code:

AddTask("mytask", {
    locks = {},
    keys_given = {},
    region_id = "farisland",
    room_tags = { "RoadPoison", "fararea", "not_mainland" },
    room_choices = {
        ["Plain"] = 10,
        ["Forest"] = 6,
    },
    room_bg = GROUND.GRASS,
    background_room = "Empty_Cove",
    cove_room_name = "Empty_Cove",
    make_loop = true,
    crosslink_factor = 2,
    colour = { r = 0, g = 0, b = 0, a = 0 },
})

Is this possible by adjusting the options, or do I have to do it in another way, for example by placing the plain and attaching the forest several times?