The graphics cards have to be the same (eg. GTX260+GTX260). You cannot SLI two different graphics card (only in certain circumstances). You also need an SLI bridge connecting the two cards together. Many of the older cards do not support SLI. You also need an SLI-compatible motherboard.
I'm not sure what you're trying to say about "automatic." SLI doesn't work just because you connect two cards into the PCIe slots. You need have an SLI bridge and enable SLI through nVidia's Control Panel.
A bridge usually comes with the motherboard, packaged separately inside the box. If a bridge was not included in your motherboard package, you can buy one from various electronics store or on eBay. The bridge is connected to the SLI ports found on the graphics cards making a connection between the two (hence the name "bridge"):
If you're planning to buy one online, make sure you buy one that is the correct length for your motherboard. If you have some trouble figuring out the length, you can buy a long flexible bridge.
A few questions. What are your graphics cards? What is your power supply?
The only game I started playing is X-Plane 9.45, and the only thing I could find on this site is for Flight Simulator X from Microsoft. One technician was saying that is mainly uses processing power and Graphic Card is not as priorty as the processor. That is not to say don't get a good Graphics card but that mainly for this type of game it is really about quad processing power. Also, SLI is not a factor either according to this post.