It works on a "piece" system. Assuming no additional entitlements due to status etc., then you would get 1 piece @ 23 kg in domestic economy. If you pay for an extra bag, you'll be entitled to two pieces @ 23 kg max each (not 46 kg distributed how you want). If either of your pieces exceed 23 kg, you will pay a "heavy charge".

Is your goal to take 30 kg in one piece? Your best bet is probably to show up and pay the heavy charge, which is probably less than the extra piece cost.