Hi gary looking at your spec i would say that loading 2 sparks in live on your system will overload the cpu, you need to up your ram a bit also this will improve performance. with output load if you load any two instrument or samples for that matter will increase output by 3db if you want to run 2x sparks turn down the levels on both by 3db to stop clipping the output.
hope his helps
ARTURIA Tester for Audiofuse, DrumBrute,spark,analog lab2,kl88,matrix12,semv,beatstep,minibrute.se,
ASC,Vcol4, Beatstep pro, VCOL5, Synclavier v.