I've never tried that, and it is possible, perhaps, to hook more than one USB keyboard up, if I had a pair of USB keyboards, I'd try it out, however I don't. Sorry about that.
On the sound: One thing you could do is find out which of Creative's soundcards will have the jacks you need, (front/rear), then search Ebay for one of these cards. They should sell very cheaply, since they aren't the newest card, and you'll get what you want without spending a fortune.
Okay, just got back from a quick search at Creative Labs website, been poking through manuals for their soundcards for you. Here's what you need:
Sound Blaster Live!, (all versions I could find manuals for on their website show a rear speaker output, which is what you need. This would be the ideal card for you, since it is still pretty much up to current standards, even though it's been around 5 years since they came out. You can still buy these new, so finding one shouldn't be a problem. I found them for just over 25$ on Pricewatch.com).
You could probably save a few buck by picking up an Sound Blaster PCI 512, or PCI 128, even, since they both have the rear jack you want, but I couldn't advise that, since the price difference, (at least on Pricewatch), is nominal.
Hope this helps. I think it'd be well worth the 25$ or so to do what you want to do. It'd be cheaper than buying a set of real surround speakers and a new 5.1 capable sound card, for sure.