How to run two separate accounts (husband's and wife's) on the same i-Pad?
Are you using the web version or the Duolingo app?
Method #1: The simplest thing would be to log one person out and back in as another if you're using either web or the app. This can probably get tiresome, but it works as-is and doesn't require you to install or configure anything further.
Method #2: If the above gets tiresome, then you would be best served by setting up different profiles on a modern browser such as Chrome or Firefox (not sure if Safari supports this too) and using the web version. This method may also require some minor technical chops (setting up browser profiles etc). Doing so would not require either of you to login/out from Duolingo or from the iPad, which works out nicely. To setup multiple browser profiles see https://help.lever.co/hc/en-us/articles/206403315-How-do-I-create-multiple-Chrome-profiles- (Chrome) or https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles (Firefox)
Method #3: Lastly, I don't know if iPad itself allows you to setup different accounts easily. That would be a good workable, option, if possible. You could then use either web or native app and not have to login/logout frequently.
Hope that helps.