How to merge live video and audio streams

I'm sending a live stream of bitmaps to ffmpeg through standardin like this:

proc.StartInfo.Arguments = "-rtbufsize 1500M -f image2pipe -r 25 -i pipe:.bmp -vcodec libx264 -r 25 C:\test.avi";

The resulting video saves and plays with no problems.

I want to add audio while the bitmaps are streaming, so I tested grabbing audio alone like this:

proc.StartInfo.Arguments = "-rtbufsize 1500M -f dshow -i audio=" + '"' + "Stereo Mix (Realtek High Defini" + '"' + " -acodec pcm_s16le -ac 2 -ar 48000 C:\test.wav";

The audio saves and plays well.

I can combine the two later, after they're saved, by using -vcodec copy -acodec copy

My question is if it is possible to merge both live streams as they are created on the fly and how to do it.