2 350 watt power supplies do not equal a 700 watt power supply. (well maybe a cheap no name 700watt PSU that could go up in flames and take all your other hardware with it)
There is second more important consideration about power supplies that GPU manufactures take into consideration when they recommend a minimum size to run a certain card, the amount of amps supplied by the PSU on the 12 volt rails. Firstly, you need enough total wattage to power your entire system, as most people are aware of. Second though, you have to have enough AMPS running at a high enough efficiency to provide a clean power stream for the GPU on the 12 volt rails. Most PSU's under 450 watts simply don't have enough power to meet this secondary, but very important requirement for mid or higher end GPU's. A 460 may run off a secondary dedicated 350psu if it is of high quality, but all in all, its just not a great idea to do this, but there are places that will tell you how, if you want to risk it. If works, great, if it goes up in smoke, well we didn't tell you how to do it here!
Just to be correct though, there indeed are secondary power supplies out there that can double up into a case. They are made to fit into the drive bays, and they are designed specifically for this purpose, to plug in and work along side your existing PSU providing the power needed exactly for this, a high end GPU or multiple GPU configurations, or other additional power needs. But to that end, if you need more power, and you have to buy one of these, you might as just as well buy an complete new PSU, you are going to spend the money anyway.