Loading Parquet Files with Different Column Ordering

I have two Parquet directories that are being loaded into Spark. They have all the same columns, but the columns are in a different order.

val df = spark.read.parquet("url1").printSchema()
root
 |-- a: string (nullable = true)
 |-- b: string (nullable = true)

val df = spark.read.parquet("url2").printSchema()
root
 |-- b: string (nullable = true)
 |-- a: string (nullable = true)

val urls = Array("url1", "url2")
val df = spark.read.parquet(urls: _*).printSchema()
root
 |-- a: string (nullable = true)
 |-- b: string (nullable = true)

When I load the files together they always seem to take on the ordering of url1. I'm worried that having the parquet files in url1 and url2 saved in a different order will have unintended consequences, such as a and b swapping values. Can someone explain how parquet loads columns stored in a different order, with links to official documentation, if possible?