I've scanned some oversize documents that are larger than my scanner. To accomplish that, I scanned them in sections and then assumed I could use Krita to recombine them into their larger wholes again.

I have not had much luck so far.

I open the first part of the document, and scale the canvas to be big enough to bring in the other piece. I open the other piece, select all, copy, and then paste it into the enlarged canvas of the first part.

I see the layer appear in the sidebar, but I can't access it, or make it visible in the canvas to properly position it with respect to the first part.

If I used GIMP it would be a simple matter of bringing the new layer forward, and positioning the graphic, then combining the layers.

How would one do the same with Krita?

Hard to say what goes wrong. It should work that way. You can't select the layer in the box or can't move it? It might help if you can post a screenshot.

You can try an alternative way, by inserting the second layer with Layer->Insert Image as layer...

I click on the layer in the list provided in the sidebar, and nothing happens. The first layer is all that appears on the screen, regardless of what I do.

I'll give the Layer->Insert Image a try.