Took a bit, but here is some info:
Go to Drive Manager ( Run -> diskmgr.msc).
For each drive right click on the part where is says Disk [#] (not the right section where is has the partitions) and go to "properties"
There will be a "Location" field with a LUN # at the end.
When a drive is attached to a Windows Volume, the lowest available LUN # is applied to the drive. The base drive is LUN 0, and each drive gets a LUN # after that. These LUN #s are static as long as the drive is attached.
Assuming there were no shenanigans with reattachments of the drives, LUN 1 and LUN 2 should be assigned in order of time.
Next, use the command line tools and run ec2-describe volumes. Search the output for your two EBS volumes, and look at the "attached" field to determine which was attached first. This should correspond the LUN 1, which you can correlate to your drive letters!
Hope this helps
Update: Looks like the section where it states Drive 0, Drive 1, Drive 2 etc, the # corresponds to the LUN #, so whichever drive is listed lower is the one that was likely attached first.