A smooth wooden floor transition between two rooms is the key to a professional result.
First, choose the direction of your floorboards and start the process in the first room. Start installing your floor in the first room.
When you reach the doorway to the other room, carry on installing across the whole width of the room. Keep going for three extra rows before turning around and filling out the remaining space of the other room.
When you have fitted both sides of the wall splitting both rooms, you can finish the installation on the other side.