Use two cameras. You already have camera 0, just dbMakeCamera(1)
dbSetCameraToImage ( int iCamera, int iImage, int iWidth, int iHeight ) for the two cameras. This will make the cameras render to two seperate images that you can then place on the screen. The problem is that you can only have one camera active at a time, so your max frame rate is essentially cut in half--but for most machines this will be negligible.
dbMakeCamera(1);
dbSetCameraToImage(0,Cam0Image,ScreenW/2,ScreenH);
dbSetCameraToImage(1,Cam1Image,ScreenW/2,ScreenH);
dbSyncMask(1 << 1);//to set the scene to render to camera 1
dbSetCurrentCamera (1);//I'm not sure this is needed, but I use it
dbFastSync();
.... then do the same with camera 0
dbSyncMask(1 << 0);//to set the scene to render to camera 0
dbSetCurrentCamera (0);//I'm not sure this is needed, but I use it
dbFastSync();
.... then paste the images side-by-side
dbPasteImage(Cam0Image,0,0);
dbPasteImage(Cam1Image,ScreenW/2,0);
dbSync();
.....I assume you know the setup for this. I only gave the code pertaining to your question.
The fastest code is the code never written.