I have a 2nd helm (Sea Star) facing up on the transom with a small wheel with steering knob. It is incredibly useful in that location, especially when playing fish, working gear and steering while surrounded by a lot of boats or avoiding heavy tidelines etc. Keep the wheel and nut greased as water will hold on top of it and you will need a really good gear puller to break it off without damaging the seals if corrosion sets in. The kicker is permanently cross tied to the main outboard, so both helms steer both motors all the time and they can still go up and down independently.
Always remember to top up hydraulic fluid at the highest helm, if you open the lower helm the fluid will flow out and make a big mess and if you were at sea you could lose steering, especially on the upper (main helm). Keep extra fluid and a small funnel on board.
Auto-steering and second helms are really different beasts with their own advantages and disadvantages. The auto helm is mostly about holding a course while the second helm for me is more about maneuvering while occupied out the back. Many times I have used it to steer around a salmon we are trying to keep from coming up on the side of the boat we still have gear down on. In my view, the second helm will cost far less, be generally more reliable and will not fail in heavy seas as some auto-steering systems can do when they get over taxed. If it were me I would add the second helm first then think about auto steering if it does not meet all your needs. Then you would have the best of both, at significant cost of course.