You can just move the location of the default library to an area which isn't User specific, i.e. not under the C:\Users\[User ID]\ directory structure, and then both User Accounts on the PC can use the same library. Not backing up on one and restoring on another, or duplicate maintenance on each. You could use the Library Clone function to clone you existing library to the location you want. See the Wiki Library Management page.
I assume that your media files are already in a non-user specific area, so the library can see them regardless of which User ID is logged in.
The only potential issue I know of is that the settings for MC are stored in the registry for each user... I think. They may be set globally. You could check that after setting up the library as above, then changing a settign under your Admin account login, logging off and logging in as the restricted user, and see if the change is there.