ok, I take it that you have the FVS124g due to its dual WAN feature? Having two 20mb connections wouldnt mean you would have a 40mb connection, you would just have two 20mb connections. Essentially, it wont work the way you have planned it as the router will only ever route traffic over one of these 20mb connections.
Now, the router itself has a hardware firewall built in, so this will slow things down straight away unless it is configured correctly, also with all that routing going on, things could become very slow indeed. The device spec only seems to mention load balancing, so if you had two 20mb connections, it will ensure you will get 290mb over both of them. On reading the spec further from the webpage, Netgear states you will only get 11.5mb throughput from the WAN to the LAN. In this case, the WAN is the internet and the LAN, well thats your network.
The answer is no, you will not get anything more than 11.5mbps going into your LAN from this device.
To confuse matters more though, you will always be getting at least 20mbps going into the WAN if you use two 20mb connections and set up load balancing. So basically there is no point having two 20mb connections, get two 10mb connections instead and see a very good connection for the money you are paying.
If this doesnt make sense, post back.