One of the main purposes I use DVD-RWs for is burning test images before I commit to burning an unchangeable write-once disc. The fact I am totally unable to do this for dual layer DVDs is stupid as hell. JVC was apparently about to release some in August of 2007 but blu-ray burners became a thing and they dropped the concept, I'm pretty sure. If it ever did make it to shelves it was in incredibly small numbers in Japan only. RIP.
When I was looking this up trying to find evidence of its existence, I found a forum thread of various people asking if they can be found.
One motherfucker in the thread really came in with "why would you want these when you can get a 500gb hard drive for so much cheaper anyways."
If I ever made a forum I would personally make it a bannable offense to post anti-answers like this. Like, legitimately, "you shouldn't want to do that" answers fill me with an indescribable rage, they are actually less helpful than not replying at all because there's a non-zero chance they'll derail the thread entirely into users trying to justify and explain why they want to do a thing when they shouldn't have to do that at all. Absolute worst forum etiquette offense.