How do I differentiate between addressing two professors with the same last name?

There are few things you can do.

1. Call each by their full name like 'Professor Ann Smith' or 'Professor Bill Smith'.
2. Ask whether you can call them by 'Professor first-name' like 'Professor Ann' or 'Professor Bill'.
3. Continue to use 'Professor last-name'. They'll either decide who will answer or ask you to clarify which person you wanted.
4. And the easiest, as pointed out by Buffy, ask the professors if there's an alternate, unique name you can use so you can differentiate between them.

In addition to the other great suggestions, you can address them as "Professor X, Sir" and "Professor X, Ma'am". That's so over the top formal that they might cringe and tell you to use their first names.

edit: as the OP points out in the comments, the following answer only makes sense in an in-person, non-Covid-19 setting. I'm leaving it up for the case the pandemic is resolved at some point.

Address them as Professor Lastname while looking the person you're addressing in the eye. The non-addressed person should take the hint from you not looking at them, that you mean the other person. If this indeed leads to confusion, count on the professors to suggest a way out.