SQL 2012 :: Blank Database Created On Two Separate Disks - Write To Multiple Files

I am testing out a blank database created over two physical files on two separate disks with one table called data which has one column called values nvarchar(max).

I filled the table up with a whole load of data and ran a select * against it. If I run Permon at the same time I can see that the read load has been spread over multiple disks as each of these disks is getting read from in parallel. If I create the same database on a single file and run the same select * again it takes much longer, proving that the read load has been distributed across multiple disks.

Now moving onto writes, this is where the confusion lies. I understand that SQL server fills files evenly until they need growing, after which it will then fill files individually until they are full in a round robin fashion unless you have trace 1117 turned on. What I don't understand is why the writes aren't distributed out whilst it is filling these file groups.

I ran an continual insert into my table with go 1000000 to monitor how the files are being filled up. I monitored where SQL server was physically placing the files as they were being inserted by running the following query:

;WITH CTE AS
(SELECT
sys.fn_PhysLocFormatter (%%physloc%%) col1,
RIGHT(LEFT(sys.fn_PhysLocFormatter (%%physloc%%),2),1) AS [Physical RID],
DATAID

[Code] ....

I could see that it would a thousand or so records into file 1, then a thousand or so into file 2, then a thousand or so into file 1 etc etc. In another words it would hit one disk, then another disk, then back to disk one to fill the file evenly. Is there any way to make SQL Server distribute the writes out in parallel so that both disks are writing in tandem?

By the looks of it, multiple disks only scale reads, as with writes only one disk is ever written to at once which is annoying. Any way to harness the write power of multiple disks?