The adjectives modify all of the following words The last two visitors: Two modifies visitors. Last modifies two visitors. The two last visitors: Last modifies visitors. Two modifies last visitors. Either is possible. 'The last two' is more likely.

Here is a good explanation of determiners including "first" and "last." https://www.englishclub.com/grammar/determiners-order.htm

The two last visitors would be a very awkward sounding way to express: the visitors who were the last two (as in from the end) The last two visitors would be the most recent to the current time. So actually, it's contextual. But I would never say 1. to get that point across. I would say it differently.

The last two visitors is correct

You can say both, they would both be correct but as a native English speaker, 2. The last two visitors sounds more natural, no one would really say the first one