First of all, I commend you for properly planning and researching your future pet’s needs before buying them – sadly many people do not. In short, you do not need to buy two wheels for them. You will find that they will go on the wheel one-at-a-time, and on the rare occasion they both try to use it they will be unable to make it function properly and one or both of them will get off. Remember that they will only spend a portion of their time on the wheel, and gerbils and hamsters are normally very good at amicably sharing their cage’s furniture.