Why would the Captain and Co-pilot both need to control the plane at the same time in normal operations? The only time that I could think of one is manually flying and doing complex maneuvering and they want to switch because of the difficulty but sounds really risky with it being much better to stabilize and then switch.
Where on earth did you get the idea that they would ever control the plane at the same time ?
Not only does it make no sense from a purely practical perspective, but also its basically a big fat no-no.
During training, you spend a lot of time on the psychology and practicalities of MCC (Multi-Crew Cooperation). To cut a long-story short, you learn the PF/PNF (Pilot-Flying/Pilot-Not-Flying) system and that's the way you work for the rest of your career.
Designated PF does the flying. Designated PNF does everything else (radio, nav, computers). PNF will also call out items that need to be cross-checked (e.g. items that get displayed on both PF and PNF monitors).
PF and PNF roles switch throughout the flight depending on operational or human requirements. The switch is very explicit with verbal confirmation of donation and acceptance of the role.
The Me-321 prototype had only one pilot. After the first flights, this and all subsequent airplanes of the Me-321 and 323 production had two. This giant plane simply required too high control forces for a single pilot, and two were needed to fly the airplane properly.
Being a glider, the 321 had manual controls despite its 55 m wingspan. Besides the second pilot, electric flaps were added after the first flight as well.
Why would the Captain and Co-pilot both need to control the plane at the same time in normal operations?
Because the fully reversible controls require more strength than a single pilot can muster.