Each switch has a set of 3 terminals, 1 on its own 2 together, the 1's on there own are the common's and the 2 together are the switched.
You have got 2 switches (top and bottom of stairs maybe) that switch 1 light, yep? all 3 switch terminals have to match in both switches.
same on the other switch for the other light, all 3 terminals in both switches have to match.
hope this helps
dotty dot dot, August 2010