One can always define such thing, but I have not seen it in theory books and I doubt it could have a good practical use.
There are types of internal combustion engines (e.g. marine-use very high power heavi duty 2-stroke diesels) that are assembled with customizable number of cylinders - e.g. see here https://www.wartsila.com/marine/engine-configurator . In these engines, your "horsepower per cylinder" will probably have some practical use.
"All factors are equal" in your Q may mean two different things:
6.0L 12-cyl makes 700HP
If it gets V10 and still 6.0L total displacement, this would mean bigger cylinders. If they are of comparable geometry to the initial engine, it will probably output the same 700HP or maybe a bit more, because of the reduced heat loss to the chamber walls. Or a bit less, because of the scale factors (like e.g. valve area growing less than the cylinder volume).
If the above engine gets V10 and proportionally 5.0L (i.e. just removed two cylinders, the crank/camshaft and the fuel delivery system reconfigured accordingly) it will have proportionally less power - just like your 583.33HP. Or a little bit higher, because there are now less cylinders competing for the throttle and exhaust capacity (if the throttle and the exhaust are kept the same).
Then again, all these things are gross oversimplifications because a great deal of factors prevent the simple scaling of the engine design.