It is not necessary to have two or more pianos in a concert venue, but interchangeable keyboards for the one grand piano.
The cost of an additional keyboard and action is small compared with the cost of a concert instrument. The DS Standard Foundation (http://dsstandardfoundation.org) makes retrofitted action/keyboards for grand pianos which can be interchanged with the conventional keyboard within minutes. Ultimately, we need three standard sizes – the conventional keyboard with 6.5 inch (16.5 cm) octave, the DS5.5®, and the intermediate ‘Universal’ DS6.0® size.
This short video shows a keyboard being interchanged during a piano competition in Dallas, Texas. https://www.youtube.com/watch?v=BAjXItVoPsY&list=PLHBn-VaaOCGcJDCJ8f-BZx5VZS-IXcxDi&index=26
