Ali Edwards has a fabulous tutorial for putting two 3x4 images on one 4x6 canvas using Photoshop Elements. If you have PSE, make sure you check that out.
If you don't have Photoshop or Photoshop Elements, here's a free way to do the same thing.
Visit www.ribbet.com, which is EXACTLY like Picnik was before it closed. It's the same platform. Looks identical and has the same features.
Sign up for a free account, and upload your two photos.
Then click on "Make a Collage."
Choose the two photo canvas (or you could play around with more photos on one canvas if you like). For two photos, I change my spacing to zero, my proportions to 40:60, and drag in my two photos.
Once you've done this, change the quality to "high" and click "done."
On this next screen, you want to crop your canvas to 4x6.
Then you can click on Save & Share. I always save the highest resolution.
Here's your finished 4x6:
Edit: I did this super quickly and didn't even notice the half of my husband in the left photo. You can adjust things like this by cropping your initial photo, saving and reuploading to Ribbet. I just didn't take the time to do that.