You could use modern optical filters (like in 3D cinema) in front of a film projector i think. Other option would be special glasses that show you the two separate images like a VR headset but with a slice of film and backlight.
The other replies are technically correct, but fancy methods or viewers aren't necessary. You can also just put the images side by side (left image on the right and vice versa) and cross your eyes until they line up.
The top lens is for focusing and framing and the bottom 2 lenses are for snapping pics
Lord help you if you mess up alignment when taking it apart or puting it back together as it will really not be fun to fix (though I was able to fix it on my other TLR, it just sucked)