Using more than one provider at the same time

Hey all. I'm new to usenet, just got started today. Everything is going well but I have a question I can't seem to find the answer to, or rather I'm probably seeing the answer but not comprehending it. As of now I am using Newsgroup Ninja as my main provider and astraweb as a backup. I understand having more than one provider is useful, but here is what I don't get. I was recently downloading some files just to test speed, adjust connections, and make sure everything was running smoothly and I noticed that only Newsgroup Ninja was downloading and astraweb was doing nothing. I use SABnzbd and have Ninja set at priority 0(the highest) and astraweb at priority 1. I know(well I think) that if Ninja can't get all of the file, astra will jump in and hopefully be able to grab the missing pieces. However, I was under the assumption that both could be downloading the same file at the same time, but that wasn't the case. So I was wondering if I could set more than one provider to priority 0 so more are downloading, thus speeding the process up, or if that would not make sense or work.