Only after some deliberate wild skids and slips to see what moved, did we work it out. A good idea ultimately, but a little too subtle!
Yes, me to, I thought the same.
The position of a stand by instrument is a compromise. There will be a turn and slip somewhere on any IFR certified panel. The designers obviously believed the location of the turn and slip was adequate for partial panel. That is the first hurdle. The second hurdle is are you better with an AI in the same position as a the turn and slip, and if you are, why didnt the designer put one there? Well he didnt put one there because of cost. They either both need to be electric on different systems or electric and vac. Either way it is not an insignificant cost.
Combine both AI with slip indicators or even just a slip indicator on the main AI and you have a fine solution. Alternatively put the slip indicator somewhere else because it is hardly essential if one or other AI is working.
However if the turn and slip is certifiable where located I dont see what difference there is if you put an AI in the same hole instead of the T and S? It essentailly does the same job - just better.