Sorry, but I doubt you’ll see dual-layer DVD-R’s out there any time soon - it’s just not possible.
Higher capacity with the newer blue laser is possible but expensive, not widely available yet, and not currently playable on stand-alone players.
As for splitting a movie to two discs, I’d never do that. Even LOTR at 3 hours is fine on one disc. The inconvenience of 2 discs in not worth the improvement in picture quality. Nothing annoys me more than having to get up and insert the second disc (or in my case, flip a two-sided disc such as Goodfellas and Stargate)… doing that once is more than enough for me… I used DVD2One to put those 2-sided DVD’s onto 1 DVD-R and am very happy.
If I want better quality when watching long movies like LOTR, I pull out my original. No worries.