Sorry I don't have this type of setup, but I was just wondering if you've tried something.
In the device properties for your drives, there is a 'media types' section. Have you tried setting up media types to disallow the LTO2 drive from reading or writing to LTO3 tapes? maybe then it would not try to load LTO3 tapes into the LTO2 drive, because it knows it wouldnt be able to read it.
Also, if you have a barcode reader and barcodes on your tapes, you can configure barcode rules along with the media types.
Let me know if this works, or if you've already tried.