I would not do that. It almost certainly will lead to trouble. But since you share all the files anyways, you could use only one account to sync the files. Your wife for example could login with your account to the sync client and use the same local folder or vice versa. She could still use her account for logging into the webgui and to sync her calendars contacts etc…
On the other hand, storage is cheap these days. Why not separate everything and share only what is necessary and use Nextcloud as it is meant to be used. In the end, you never know what things will get chnaged in the future or whether you will find a use case that could cause problems with this configuration.
Or you could use the new Virtual Drive feature. Virtual Drive, only downloads the files that are actualy used. That’s probably the best solution if you want to safe storage space on your computer.