OP, if you're just wanting to share your 3G SIM between two iPads from time to time, check with your carrier if that's OK. I have AT&T and checked and they said it was fine to swap between my iPads. I just use one iPad at a time on 3G, but I had them note that I checked on my account, because carriers have the right to cancel your service if they think there's any funny business going on. For me, that would mean risking my unlimited plan.
Some carriers allow tethering your 3G from a smart phone to an iPad, but some charge and some don't. If they find out you've been tethering against terms of contract, they can cut you off, too. The contract terms are weighted in their favor, so all they basically need to see is a spike in data use.
Sharing 3G to download might not work in cases if content is over 20MBs anyway. For that you'll need wifi.
You also could look into getting a personal hotspot, but that's not necessarily worth it unless you plan to be in proximity all the time, because the hotspot has to be nearby.