For Unity 2019.4 I needed to set one camera as an overlay camera via...
"Camera Component" --> "Render Type" --> Overlay.
Secondly, I needed to take the other camera and add a stack element in it's Camera Component (at bottom) then select my Overlay Camera to add to the stack.
The usual way is:
Render opaque objects first
Blending disabled (this does not save memory — the render target still has just as many pixels with just as many bits of colour information — but it does save time spent doing the blending calculation)
Sorted front to back (so earlier objects tend to occlude later objects, and you can skip drawing some of the ...