Is it acceptable to email the authors of a paper for the slides they used while presenting it in the conference? I've noticed that while there's no alternative to going through the actual paper in detail, it often helps if I've gone through the presentation based on the paper, which the authors had used while publishing their papers. (Mostly because, the few presentations on TCS papers that I've seen have good geometric examples/illustrations which may have been instrumental in getting the intuition in the first place, but the brevity required by CS conferences mean very few of them make it to the published paper!)

While a few authors graciously maintain the links of such slides on their web-pages, most do not - and I haven't even seen a single CS conference maintaining such documents on their web-page!

So, I've the following questions:


Since I'm ultimately interested in understanding their work (and extending it if possible), is it considered OK to email the authors for the slides (if they still have it)? 
If it is, do I have to justify/explain why I want the slides, or would just a polite request do (with minimal explanations on my part to keep the mail short!)? 
Would making such a request more than once make the author feel offended, and/or make them think that someone who can't read papers at the highest level without "assistance" isn't exactly someone worth helping out?
If there are 2 authors listed, without any implications about who is the corresponding author, should I mail both of them at the same time (which may lead to both of thinking the other would be responding), or separately (which may lead to duplication of efforts!)?


I'm not sure what to tag this query with - feel free to re-tag/add more tags as you see fit!
It's never a problem to ask. In particular if you are interested in extending their work, they should be interested in providing you with more information. Perhaps the extension will happen in collaboration with them, but at the very least you're going to cite their paper. In my experience, views on sharing slides vary, but the objections I've heard related to unpublished material; I understand that in your situation, the material is already published.
I would explain why. It's polite to do so. But do keep the mail short. You could write that if they're interested, you can provide more details on your ideas; like that, you don't swamp them with unrequired information, yet you're open from your side in stating your ideas. Such an offer shows that you trust them, and as they need some trust in you if they share their slides, that is probably a good idea.
I don't think they would feel offended. 
Depends on the field. If there is no clear first or corresponding author, I would contact either one of them. They would probably either reply or forward the e-mail to the other author.