IPAD - Email

You cannot - there is no way to prevent access to the email application by anyone using the device,

Sorry.

Maybe it's better to make a www.Gmail.com address, it free and You can choice the name You want and then use it for all other mail then privat.
You can add this Gmail address to your iPad.

Edit: Sorry I see know that You not want that You wife can see your mail, it's my English 😉
Then You have two options, if You trust each other, then You don't have to look in each others mailbox, otherwise You must buy and iPad for your wife 😉

we would not want the other person to see our own private email - can you do this and how ?

This can only be done via webmail where each person has to log in via a browser. Not all webmail systems work well with iPad browsers.