With all due respect vinsome, it's actually not an awful suggestion, and quite logical when you think about it.
To put it simply, you can't have both 2G and 3G on one SIM for differing service, because the network knows to connect to either the 3G base station OR the 2G base station according to the SIM ID number - the only way around this is to use the 3G services you want to use in one location, and then when you want to browse the Internet, go to an area of the country where 3G service is not offered by your carrier.
Other than that, you are, unfortunately, SOL unless and until carriers start charging the same rates for data irrespective of whether you're surfing at 2G or 3G speeds.