Interesting question. But I would think that the anti-vibration, anti-jitter, etc. tech in a modern burner would be able to compensate even if there was a small amount of vibration.
I guess I would try to be sure that the drives do not actually come in physical contact with each other when you attach them to the drive cage, and then make sure all 4 holding screws are tighly in place.
My bud doesn't have fan clips on his rig - he screws 'em right to the case wall - and his case seemed to vibrate overall a lot more than mine, which has the plastic fan clips between the case and the fan. He opted to get one of those "rubberized vibration dampening" kits, which actually came with small rubber washers that go under each screw-head as you fasten them, and believe it or not, it seems to have done the job nicely.
I actually thought it might be cool to get some of those tiny rubber washers for my rig, but I can't seem to find any excessive vibration. Still, the concept sounds cool. Maybe if I could get glow-in-the-dark rubber washers...
Here is a thought. Maybe try spacing your hard drive as far away from your optical drive as you can and do a couple of high-speed burns and use Nero CD-DVD Speed to do a Disc Quality check and take screen shots of the results.
Then try moving that same hard drive right next to your optical drive, make a couple of high speed burns, run the CD-DVD Speed Disc Quality check, take screen shots of the results and compare.
That could be kinda fun, and you could share the results with fellow users.