Fortunately, it's rather difficult (doing a conversion like this) to get the front axle at precisely the correct height, and so one rear axle or the other is carrying most of the load, and the other is just touching the ground, and therefore it turns fine. I did put a heavy load directly over both rear axles, which put them both in full hard contact, whilst at the same time slightly relieving the front axle - and yes - there was a tendency to carry on!
None the less, the principle is a potentially useful one