How can I configure my iPhone and iPad so that if I delete an email from one it also deletes it from the other

Ideally, ensure that the Mail accounts that you have configured on your iPad and iPhone are using IMAP. Check with your Mail provider that your account can use IMAP - and verify the necessary settings.

You can find more information about setting-up your Mail on both your iPad and iPhone here:

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

Make sure both devices are logged into the same Apple id.