Depends how.
The PictureBox does have the BackgroundImage property, so you could kludge something together like that.
Otherwise, it really isn't painful to put two images together yourself like so:
private Bitmap LoadTwoImages() {
Bitmap image1 = GetBitmap1();
Bitmap image2 = GetBitmap2();
Bitmap combined = new Bitmap( image1.Width + image2.Width, image1.Height > image2.Height ? image1.Height : image2.Height );
using(Graphics g = Graphics.FromImage(combined)) {
g.DrawImage(image1, 0, 0);
g.DrawImage(image2, 0, image1.Width);
}
return combined;
}