The UD3 does not originally support Nvidia SLI configuration.
But as BIOS Modders have learnt it is possible to enable the BIOS to understand and allow the UD3 to become SLI capablehttp://www.bios-mods.com/forum/Thread-Request-SLIC-for-GA-P55A-UD3--1824
However, their UD6 supports both CF & SLI
When you put in two of your Nvidia cards into the mobo they will work, but they will not be able to function like a set of cards, there will be no load sharing between the two. And the driver may just not allow you to setup the SLI configuration in the NVCP.
Basically , the BIOS of this UD3 board does not have the instruction set to understand what to do when 2 Nvidia cards are put in, so it is going to treat them as two VGA Adapters and nothing more, thereby, when you use them , you'll get the power of just one card, it's like putting a sound card in a deaf man's system.....
The second one is going to be just useless and draw power.