The command "board" will switch from the schematic to the board and therefore could have a key set up to do it. I cannot find an equivalent command to switch back. It may be possible with a ULP.
Another possible solution - ALT+0 or ALT+1 or ALT+2 or ALT+3 etc allows you to select windows - control panel, library, board & schematic. The only snag is that the exact number to press with ALT depends on how you opened your files so ALT+1 may be the schematic or it may be the board or even a library.