No. In order to do this you would have to modify the OS (which for iOS you won't likely be doing) to create a load balancing driver.
The benefit could be a minor increase in performance, but that increase would be minor. Likely the work and headaches of such an endeavor would not be worth the minimal gain.
Not too different of an answer though. You still need a load balancing driver to make it possible for the system to see AND use 2 internet connections at the same time. This could theoretically be done on the PC, as you can write drivers for the PC.
But the upside of doing that would hardly be worth it (3G is incredibly slow compared to your Wifi/broadband).
If you're just talking about tethering, so you have internet access when you're away from your wifi, then that's a different story. I think most iPhone providers either block tethering completely or require you to buy a tethering package. There are jailbreaks that bypass this. (I've used Android for the last 2yrs, and lost touch as to the latest for iPhones + tethering). But I've got a feeling this isn't what you're referring to...
My iPhone 4S can be used as a hotspot through USB without buying anything and actually has a download speed equal to my wifi and the upload is a little better, and the ping is also the same. What I was hoping for was to be able to use both at the same time to just improve the internet. As i share the wifi with 12 other people it can be a little slow sometimes...Well most the time
Wow, that's some slow broadband. If it's that bad, just use your iPhone as your broadband. Or buy a better package from your ISP.
But unfortunately I've not seen anything created to enable the loadbalancing we're talking about. Something could exist in the vast interwebs, but I've not seen it. And I wouldn't expect it to work well (load balancing is a pain in the *** at the best of times )