Can i run a GTX260 and a GTX580 at the same time?

As a PhysX card, yes, possibly for F@H. That's about it.

should i use it?

That's up to you. Since you already have a GTX 580 I doubt the 260 would provide much improvement in physX games over just using the 580 for graphics and physX. Not only that, but there are very few games that take advantage of GPU accelerated phyX while the 260 generates a considerable amount of heat and will use a considerable amount of power. I probably wouldn't if I had a 580, but it's up to you.

There was a test done that if you have a weaker card as a PhysX card it could hurt the performance (Test done on a GTX 580). Usually a good PhysX card would be the same generation or just a previous generation. And this boost would only be about 10FPS or so in games that use PhysX. Most games dont utilize PhysX.