I'm kind of confused about what result you wanted to have.
Anyway, locking the alpha channel should be completely useless in this operation: Locking the Alpha channel (the box you ticked, screenshot 2) basically means that you cannot edit any transparent part of your image. You prevent the transparency channel to be altered by any tools (for example, if I have a photo in front of a transparent background and I lock the Alpha Channel, I can paint all over the image with the Paintbrush Tool but I will only be able to paint over the photo, and NOT the background.)
Your photos don't even HAVE an Alpha Channel, so there shouldn't be any use for that option for your problem.
Or, as the official Gimp docs say:
“Lock alpha channel” setting
In the upper left corner of the Layers dialog appears a small checkbox that controls the “Lock” setting for the transparency of the layer (see the figure below). If this is checked, then the alpha channel for the layer is locked, and no manipulation has any effect on it. In particular, nothing that you do to a transparent part of the layer will have any effect.
I'll make the same operation and document every single step:
- Start Gimp
- Open two images as seperate layers
- Click on the chain symbol next to both layers
- Click on the resize-tool
- Cick on the top layer
- Drag and drop it to a smaller value
Works like a charm. Both layers get resized to the correct size.
Try it again and use my steps. If all of that doesn't work, get Gimp 2.8 and put the two layers into 1 layergroup which can be resized as well.