I think you've answered your own questions. You tried burning to two drives at once and it screwed up. Can't speak from experience, as I've never tried to do this, but it is something I wouldn't even attempt. Why waste disks? Or risk corruption of data just to save a little time?
If you are serious about using these dvds as an archive, I would use single layer disks. If your +R Sony disks are Taiyo Yuden, this will be as good as it gets. Dual layer disks, even Verbatim, are an unknown quantity for archival use. Even with the TY single layer, I would periodically check the condition of the disks, and plan on reburning them at 4 to 5 year intervals.