Just sharing some experiences...
When burning 2 discs at a time with Nero, the program has a column titled 'Read Position' that is just after the 'Buffer Level' bar graph. See screenshot. It indicates in % how much on disc is 'ahead' of the other. I think it is % of the 80MB buffer, since it can move about 10%/second when moving quickly, and that would correspond to about 10% of 80MB = 8mb/sec, about 6x burn speed, so definitely in the ~4x-8x burn speed range of the 8x burn strat.
The discs usually start out at 0% Usually it stays around 10-20%, often more and often less,(sounds funny, but there is no consistent spread) but never at 0% the whole way thru and never over 100%(which I think means the buffer).
Another peculiarity is that when the data portion of the burn is done, Nero closes one disc, then the other disc. It does not burn the 'closures' at the same time to each disc. That adds about 40 seconds to the total burn time of dual@8x burntime.
The burn results are usually a little worse for dual-disc@8x than single disc@12x, but the results are still acceptable. 90% of the discs I burn get a quality score of 96-98@single-12x, and 94-96@dual-8x. I haven't tried 4x, because 2 single discs @ 12x would take about the same time, within a minute, so no speed benefit and a doubtful quality benefit.
However, everyones results will vary, and I feel that I get pretty good results with my setup, so you may not see as good of results as I get. Maybe you'll get better results...I hope you do! As rdgrimes states, 4x is probably safer to start out with,