My wife and I have a working version of this. Until a few years ago, I was handling everything, mainly because I read Japanese better than she does. She realized she'd be up a tree if something happened to me, so she brought it up and we shook up our system. Two things simplified the situation:
1. A password manager (we use Lastpass). We each remember one master password that gives us access to all of our online accounts in one place. So now we each have access to everything money-related we do online, and it's all there like a list. If using a computer at all is a no-go, this won't work, but if you can get a person to remember one password (and where to enter it), it might help.
2. A chart for tracking income and expenses, day by day, by hand. We used to track income and expenses only on an Excel spreadsheet on my computer. Now, we write everything down on the chart. At the end of every month, she enters it into the spreadsheet, I check, she double checks, and we discuss. Going analog actually made this system better - we keep the sheets around, so we have a written record of what kinds of expenses usually occur when. Something a surviving spouse can refer to if it comes to that.
Now, she's more in charge of these things than I am, which is great for me because I can focus more on work. But we both have access, and each of us can easily find all the information if and when the SHTF.
