There is more than one way to do it, If you just want to do it with blue gems like in the game, you can use a "Counter" put the counter on 2, and make each gem in an eyesocket decrement the counter. When both are full you'll get the door open.

Any of the more complex puzzles you will want to make will need counters, For example putting multiple torches in holders, or removing torches or a combination of both. Pressing a special combination of pressure plates and so on can all be done with counters.

Okay. Thanks

Can you explain it in more detail? Like first you place... then you... And so on.
This is kind of new for me.

I havent watched these, but I understand that a lot of people have found them very useful, They're a full tutorial on making a dungeon by Komagb, start at No1 and go though, you'll learn a lot

http://www.youtube.com/watch?v=2YhbRDX4wsM