Combo passes are valid for 30 days for a 2-attraction pass, and 60 days for a 3 or 4-attraction pass from your visit at Madame Tussauds Sydney. So, you can do it all in one day or prolong the fun over the full validity period depending on your pass.
You will need to book your time slot at each attraction to guarantee your entry, you can book these here. Please make sure you check the opening and closing times for the other attraction/s to avoid disappointment.