b.) If there is a system which enables different boards to work with each other then there are still likely to be restrictions such as having them of the same generation / capabilities. Whilst differing feature & shader capabilities could be worked around other elements such as shader precisions, output precisions, filtering algorithms, FSAA capabilities, etc., could potentially mean that the portions of the display may not be exactly the same - whilst the differences may be negligable between two boards when comparing two fully rendered images side by side, having the two render parts of the same frame or alternate frames these things may stand out much more, ergo this is a bit of a no-no.