Providing there is enough slack you can just match the pairs from each severed half. You can just twist them together but it is better to solder or use a breadblock. If twisting together obviously you will need to insulate with tape each connection from the others. I usually then wrap a piece of tin foil around the whole join to shield it from stray noise. If there is not enough slack then you will need to make two joins with a piece of spare cable. It might be cheaper to buy a short patch lead and cut the plugs off if you don't have a spare piece of cable. Do this when the cable is unplugged or everything is switched off.