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I am looking for a command line tool that can download multiple urls with multiple threads e.g.
wget2 -n 5 http://stackoverflow.com/ http://askubuntu.com/ http://bobo.com/
Where -n = number of threads. I have come across Axel, but when I give it multiple URLs, it only downloads one.
I will be downloading HTML files.
Aria2 is the best solution for this if you want CLI. Aria2 supports multiple connections, multiple threads and multiple sources.
Another benefit of Aria2 is that is works as a plugin for uGet so you can use the power of Aria2 with a nice easy to use GUI.
Aria2 - CLI - http://aria2.sourceforge.net/
uGet - GUI - http://ugetdm.com
Update: based on OP's batch needs
uGet supports batch downloads via .txt, .html, clipboard and many more methods. While admittedly not CLI, I think it solves the problem quite well. I created a video tutorial to explain the various methods, the GUI has changed since this recording but the functionality is still relevant.
All of the above and linked suggestions do not take two unique URLs. They only take URLs that are mirrors of the same file.
I've found a few programs that do this:
The best is puf (apt-get install puf), use puf url1 url2 etc.
Then there is HTTRACK, which requires a lot of tinkerings and has some limites I can't get past (speed and connection limits)
DownThemAll for Firefox is very good if you don't need a command line app.
UPDATE
I've since found puf has a tendency to crash. The best solution is to create a .txt file with URLs on new lines, e.g.
http://google.com/
http://yahoo.com/
Save that are urls.txt (for example) then run the command:
cat urls.txt | xargs -n 1 -P 10 wget -q
-n specifies to select each line from the file
-p specifies the number of URLs you would like to download in parallel.