I have a similar situation.
I always re-boot before starting a burning session using Nero, otherwise the results are too varied and unpredictable. To be clear here, I just mean before a burning session, not before each individual burn. If I run other programs, or have been doing something else on the computer, I will re-boot before starting a burn.
That having been said, I can get perfectly consistant results all day without one re-boot, if I burn one disc in the Pioneer with Nero, whilst testing the previous burn in the Lite-On with K-Probe at the same time.
I haven't noticed any issues regarding which program starts first or last. But I do always start my K-Probe scans before I start the next Nero burn, to allow for the Lite-On to spin up properly and start sending data through. It seems to hog the channel a bit at the begining of the test.