In this case, you have to expressly map all needed streams, as the auto mapping will only pick up one audio stream. So,
ffmpeg -f dshow -i audio="Stereo Mix (Realtek High Definition Audio)" -f dshow -i audio="Microphone Array (Creative VF0800)" -f gdigrab -framerate 10 -video_size 1920x1080 -draw_mouse 1 -i desktop -map 2 -map 0 -map 1 screen.avi