以下是一些示例数据：

days = 365*2
date = seq(as.Date("2000-01-01"),length = days,by = "day")
year = year(date)
month = month(date)
x1 = cumsum(rnorm(days,0.05)) 
x2 = cumsum(rnorm(days,0.05))
df1 = data.frame(date,year,month,x1,x2)

我想同时聚合来自df2数据框架的x1和x2变量按年和月。以下代码聚合x1变量，但是是否也可以同时聚合x2变量？

### aggregate variables by year month
df2=aggregate(x1~year+month,data=df1,sum,na.rm=TRUE)
head(df2)

任何建议将不胜感激。