Pushing childSymbol in array of two dimensions

I try to create an array with two dimensions, but I don't know how to assign the new childSymbol inside of array in a concrete position.

With one dimension:

var aPeces = []:

var symPeca = sym.createChildSymbol("symImatge", "Stage");

aPeces.push(symPeca);

With two dimensions:

var aPeces = []:

var symPeca = sym.createChildSymbol("symImatge", "Stage");

aPeces[4,6]=symPeca; //??????????????

Thanks.