J-L:

Open the floating window by using the "Window" menu at the top. In that menu, select "Movable piano roll window". A floating piano-roll opens. Drag it wherever you like, including the secondary monitor. (It will not persist on a second monitor if you restart BiaB.)

It will be good if somebody else knows a more direct way to get a floating piano roll open. The "Window" menu gives no shortcut keys.