Rendering problem while using dual Graphic card

Recently I have added two GTX560Ti graphic cards (with SLI connector) to my system. I tried using them to render my work. Although I was happy to see that the rendering was very fast, I faced a problem too. Some frames in between were not totally rendered. See the samples below

I faced this problem only while rendering using both the graphic cards. It doesn’t appear while using single graphic card.
Please help me to fix this issue… Thanks guys!

When using two GPUs for compute functions it is usually recommended that you NOT have them in SLI (Blender can still use both simultaneously). Not sure if this is the issue here but worth checking out.