Position the plantation blinds over both of the windows with the help of an assistant or two. Arrange the blinds evenly so that the support board on the side of both blinds can share the center frame between the windows. Set a carpenter's level on top of both windows to make sure that they are level, as well as evenly spaced.
Mark the locations of the support board screw holes in the frame on either side of both windows. Pre-drill all the marked holes in the frame using a drill and a drill bit with a significantly smaller diameter than the installation screws.
Place a 3-inch wood screw into each pre-drilled pilot hole. Tighten the screws halfway until all the screws have been installed in the support boards. Complete the installation by tightening the screws fully and testing the blind operation.
Hold the two blinds in place with the help of an assistant. Butt the end of one of the blinds into the support board of the other and arrange the blinds so that the intersection of the two blinds looks even and the blinds are even on the other sides of the windows. Set a carpenter's level on top of the blinds and ensure that they are level.
Mark the location of the intersection between the blinds with a vertical pencil mark. Remove the blinds and stand them upright on the working surface with the help of an assistant. Reposition the blinds based on the previous marking and make new markings where the holes from one of the support boards align on the other support board.
Pre-drill new holes into the markings. Attach the blinds together with 3-inch wood screws drilled through the manufactured holes in one support board and into the newly drilled holes in the other.
Reposition the blinds against the corner window and mark the location of the support board screw holes on the other ends against the frames of the window. Remove the blinds and pre-drill these holes. Attach the blinds to the window frames with 3-inch wood screws through the support board holes and the pre-drilled holes.