Wouldn't the resultant bitmap be 320 x 120? I'm also assuming the depth is the same on both. Allocate the size of the bitmap and use trusty old BitBlt.
// leftDC and rightDC are the two drawing contexts on the images to combine.
// memDC is the DC on the combination image.
BitBlt(memDC,0,0,160,120,
leftDC,0,0,SRCCOPY);
BitBllt(memDC,161,0,320,120,
rightDC,0,0,SRCCOPY);