Mixing Burners on same machine

I am using two NEC ND-3650A SATA DVD burners in a Dell 9200. These burners rock so I decided to buy two more and put them in an external enclosure and use eSATA between the box and the PC. I burn a lot of the same DVDs, so burning four at once is a big plus.

The problem is that the next two that arrived were Toshiba Samsung TS-H553 drives. Same Dell part number but different OEM drive :a

At any rate, I have always been told that if you plan to burn multiple disc at the same time, you should always have the same speed and model of burners. These are bothe 16X but I need to know if I am asking for trouble by having two different models?

Also, so far I have been using Roxio 8 to burn two at once and I hooked up a SATA CDRW to check that it will see more than two and it did. Does NERO also burn multiple (more than two) disc at once?