I have tested it, they do not stack. For future reference, it apparently does need to be a separate purchase at the appropriate price point for access to the files. So if I spent $1 for the .32 version (in this case 21 days ago), then I need to spend 2 more dollars for access to the .54 version. Not 1 + 1, it has to be 1 + 2.
It's not that big a deal. I'd rather have learned this from your game, than from another one. But that is a flaw in the design of itch.io it seems.