I've got two 5x7 framed prints that I want to hang (together) on a wall that is about a foot and a half in width; what should the placement be? Beside each other, one on top of the other, on a diagonal?

What ever you think looks good. I personaly would go side by side.

Is the layout portrait or landscape or one of each? If they are portrait, I would put them on top of one another. If they are landscape, I would put them side by side. If you have one of each, side by side. Although, I never hang anything in even numbers. I’d be sure I had three pictures. More aesthetically pleasing. But that’s just me.

Any art is best viewed eye level. I say side by side.
Having said that, it’s your cake man, eat it any way you like! ;)

They’re two black and white (vintage) portraits. I’m considering on picking up a third.

If you get a third, then place them side by side (by side). But as said above, place them a bunch of different ways, and see what suits you best. You really can’t do it wrong.

I would say based on the narrowness of the wall, on top of one another or on top of each other diagonally.

Vertically, because of the narrow wall.

Vertically, especially if you get third.

Vertically

If it’s 2 vertically oriented prints, side by side… even on a narrow wall. 2 horizontally oriented prints, one over the other.