/**
 * This class can be referred to as your AuroniumCrystal
 */
public class ItemFirst extends ItemModded {

    public ItemFirst(String unlocalizedName) {
        super(unlocalizedName);
    }

    @Override
    public ItemStack onItemRightClick(ItemStack itemStack, World world, EntityPlayer player) {
        if (player.inventory.hasItem(ModdedItems.second)) {
            ItemStack itemStacks[] = player.inventory.mainInventory;
            ItemStack secondItem = null;
            for (ItemStack stack : itemStacks) {
                if (stack != null && stack.getItem() == ModdedItems.second) {
                    if (secondItem != null)
                        break;

                    secondItem = stack;
                }
            }
            if (secondItem != null) {
                if (!secondItem.hasTagCompound())
                    secondItem.setTagCompound(new NBTTagCompound());
                if (secondItem.getTagCompound().getTag("someTag") == null)
                    secondItem.setTagInfo("someTag", new NBTTagInt(0));
                else {
                    int amount = secondItem.getTagCompound().getInteger("someTag");
                    secondItem.getTagCompound().setInteger("someTag", ++amount); // In the previous example I made a minor mistake here
                }
            }
        }
        return super.onItemRightClick(itemStack, world, player);
    }
}

/**
 * And this class will represent the AuroniumHolder
 */
public class ItemSecond extends ItemModded {

    public ItemSecond(String unlocalizedName) {
        super(unlocalizedName);
    }

    @Override
    public void addInformation(ItemStack itemStack, EntityPlayer player, List toolTip, boolean flag) {
        if (!itemStack.hasTagCompound()
                || (itemStack.getTagCompound() != null && !itemStack.getTagCompound().hasKey("someTag")))
            return;

        toolTip.add(String.format("Value: %d", itemStack.getTagCompound().getInteger("someTag")));
    }

}