If, for example, i have a joint of meat that is 2kg and requires cooking for 2 hours, would the cooking times be halved if i cut the joint in half and placed them side by side in a roasting dish in the same oven ?
Yes , but a large joint cooks 'better' in terms of flavor and tenderness than 2 small joints. 1Kg is hardly what I would call a 'joint' more like a large portion .
This DOES NOT apply to microwave cooking, which is down to the weight not the size of the piece of meat.