You will of course need to choose how you go about designing your sites. You may want to decide first which site you think will get the most traffic, the French or the English? If you think the English side will get more traffic, then you can set that up as being the default page and make visitors click over to see the French (or vice versa).
Assuming that you think that English will be more popular, here's what you would need to do next: you'll need to create a new WordPress.com site for your additional content to be on (so in this case, as an example, andrebourdagesfr.wordpress.com). You can then transfer your French content over to that site and purchase the mapping for fr.andrebourdages.com and set it up on that site.
You can then set up a static front page on andrebourdages.com which welcomes visitors and directs them to visit the French page if they would like or click through to the English pages by using the Static Front Page instructions right here: http://en.support.wordpress.com/pages/front-page/
That should do it for you!
Of course if you would definitely like to have completely different sites for en.andrebourdages.com and fr.andrebourdages.com, then you will need to do effectively the same thing as I described above, just twice. Create a new blog for each one in your same user account and purchase the mapping for each and then in your normal andrebourdages.com site direct users to those sites on a static front page.
To answer your question, there is no discount for purchasing multiple domain mappings, I'm afraid.