双人共乘：邀请上车与同步下车

-- 错误：各自独立上下马，乘客悬空
function board(a)
    setVar(a, "onMount", true)
end

local function board(a, b)
    setVar(a, "riding", b)
    setVar(b, "riding", a)
    syncMount(a)
end
local function dismount(a)
    local b = getVar(a, "riding")
    if b then
        setVar(b, "riding", nil)
        syncMount(b)
    end
    setVar(a, "riding", nil)
end