Does the NeroAPI support burning to several drives in parallel? NeroCmd does not seem to support this feature via its command line, whereas you can tell "Nero Burning Rom" to do so. I'm using NeroSDK 1.04 and Nero V184.108.40.206.
You cannot initialte two burns at the same time in the same process with NeroAPI but it should not be a problem to start two processes. Try burning with two separate instances of NeroCMD.