I have 3 DVD burners and 1 DVD rom in my case. I have ripped 4 movies at the same time before. (Now that my NEC 3500 is the best burner in my group, I usually restrict myself to no more than 3 at a time, just to save on wear and tear to that burner.) There is some speed degradation, ie the disk in drive X would rip a little bit faster if drives y, z, and t were't also ripping disks, but the total time is shorter than ripping 1 disk at a time. (I also have 3 hard drives - 1 with the operating system and 2 that I use for backing up movies.)
There is little total speed advantage to encoding more than 1 movie at a time. Just to use round numbers, if 1 movie takes 1 hour to encode, then 2 movies may take 1 hour, 45 minutes to encode. However, I do encode multiple movies at the same time to save on computer/huiman interaction time. If I have 4 movies to encode, I will start all 4 movies at the same time, then go to bed. By morning they are all done and I didn't have to stay up an extra 3 hours to start the next project.
I haven't experimented much with burning multiple disks at once. Although I have 3 burners, it seems like I only have 1 really good burner at a given time. My Pioneer 107 was my previous awesome burner, but I think I wore it out. It still reads through anything, but it now produces a mysterious dark ring in the burns that always have errors. I think you might be successful if each burner is on a seperate ide channel, and both of these are seperate from your hard drive ide/sata channel and the hard drive isn't your system drive. But that's just a guess.