As I said:
you can always change priorities for downloading, decoding & par2/unrar threads for each instance of Alt.Binz via Setup > Miscellaneous #2 (not sure how well this would work, as I haven't tested it).
Why not try it and let us know? Maybe Alt.Binz will just fall over. As for your scenario, I'm afraid it still doesn't really make sense to me. Why split your speed between queue items? What's it supposed to accomplish?
To illustrate: say you have 2 files, each the same size, that would take 10 minutes each to dl at max speed (the limiting factor being your bandwidth).
With consecutive downloads in a single queue at 100% speed:
@ 0 mins
file A starts to dl
@ 10 mins
file A completes & unrars
file B starts to dl
@ 20 mins
file B completes & unrars
With parallel downloads and 50% speed/threads each:
@ 0 mins
file A starts to dl
file B starts to dl
@ 10 mins
file A is half complete
file B is half complete
@ 20 mins
file A completes & unrars
file B completes & unrars
Parallel downloads with file A at 80% speed, file B at 20% speed:
@ 0 mins
file A starts to dl
file B starts to dl
@ 12.5 mins
file A completes & unrars
@ 50 mins
file B completes & unrars
Wiser heads please correct those sums if they're wrong, I'm terrible at maths