Shawn, the calculation for parallel impedances is as Brian illustrated(the sum of the reciprocals of the individual impedances equals the reciprocal of the combined impedance), using the rated numbers of 6 and 8 ohms. Of course those numbers for impedance are simply an approximate rating and they vary greatly with frequency. In any case, impedance is rarely a real problem with equipment of reasonable quality and shouldn't lead to overheating and shut down at typical sound levels. It's unlikely that the parallel connection would cause a significant problem with your Onkyo and the assumption at this point should be that it'll work satisfactorily.
