import pandas as pd

data = {
      "case1": {
            "aaa": 1222,
            "bbb": 1333,
      },
      "case2": {
            "ddd": 1444,
            "ccc": 1555,
      }
}

case1_df = pd.DataFrame(data["case1"], index=[0])
case2_df = pd.DataFrame(data["case2"], index=[0])
# 生成多级表头DF
df = pd.concat({"case1": case1_df, "case2": case2_df}, axis=1)

# case1_df = pd.DataFrame(data2["case1"], index=[0])
# case2_df = pd.DataFrame(data2["case2"], index=[0])
# df2 = pd.concat({"case1": case1_df, "case2": case2_df}, axis=1)
# df = df.append(df2)
# df.to_excel("111.xlsx")

# 读取多级表头数据
df2 = pd.read_excel("111.xlsx", header=[0, 1])
# 向原多级表头的excel追加数据
df = df2.append(df)

df.to_excel("111.xlsx")