How to play nice.....

Can anyone give me any advice on this situation????? Here's the scenario: Nicholas builds a design with legos, Joshua wants to destroy it, Nicholas gets upset, fight ensues, mommy breaks it up, repeat. Joshua really wants to play with the legos too, but Nicholas doesn't want him too. So, Joshua decides to get out the dominoes and make his own creations with them. Boys are playing happily, but then Nicholas wants to "help" Joshua make the "correct" shapes he says he is making. I try to get Nicholas not to help him since Joshua wants to do it on his own and doesn't mind that the shapes aren't really correct. That's the learning process right? So, Nicholas then decides he will make his own creation with the dominoes, but Joshua does not appreciate him taking the dominoes from his creations, so a fight ensues again. Why doesn't Nicholas understand why Joshua was upset? Didn't he get upset when Joshua tried to take the legos? Now, I appreciate him trying to help him learn, but at the same time he shouldn't take the dominoes away when Joshua says he doesn't want help. Oh the joys of parenthood. Anyone know how I can combat this? Burt says to put them in seperate corners with their toy of choice. Hmm, maybe.