Yes, I know what you mean and am sorry for the frustration. It’s been on the list to address for a while; because of the way the viewer is built, it’s a much more thorny problem than it would appear at first glance.
One thing I can suggest, lacking the actual feature to hold the zoom, is to take advantage of multiple tabs:
1) make a new tab
2) Pick the same album or folder in the second tab
3) Set the selections in each tab and set the zoom as you like
4) Then you can use Cmd-Shift-] and Cmd-Shift-[ to switch tabs and thus show the two images.