two email accounts - how to distinguish easily

Yes, it’s very easy to differentiate between multiple accounts. You simply need to edit the Description field for each email account in Settings:

Settings > Passwords & Accounts > [target email account] > Account > Description - edit this free-text field with a suitable name

The text that you enter in the Account Description field will be shown in the list of Mailboxes within the native Mail App. If, for example you named your two accounts as “Alice” and “Bob”, these descriptors would appear in the list of accounts in Mail.

The account description normally defaults to the email address, but can be changed as necessary; this fireld does not affect the email address associated with the individual account.

I hope that you find this information to be helpful in resolving your question.

You can look for third party apps, but most that I’ve seen have a single ‘send from’ option

I would suggest looking for a third party app (I use Edison mail but there are plenty of others) and you use one, he uses another.

Keep them totally separate to keep them from getting mixed up

You would also want to set the default account on the iPad to your account, then any mail you send outside of the mail app would go from that account, as well as any mail you send in response to a mail in your account. The only time you should send from your husband's account is if you were responding to mail received in that account.