I would like to make a statement about two people. One of them is dead:
He and his wife {are/were} great scientists.
Are probably indicates that both of them are still alive. Were indicates both are dead or at least are not great scientists anymore. If that is correct, will rewording be better? For example,
He was a great scientist as his wife still is.
If you can, start with the living:
Carol Smith is a great scientist as was her late husband, Bob Smith
If not, try something like this instead:
Bob Smith, until his untimely death (2012), was an astounding scientist as is his surviving spouse, Carol Smith.
If one of the couple is still alive but the other is not, then it's awkward to just use are or were. Either way would be misleading, and you'd have to follow with some kind of retraction.
Both the Professor and his wife are great scientists, although he died last year.
Both the Professor and his wife were great scientists, although she is still alive.
You see? Awkward.
Instead you ought to rephrase the sentence. One way to do this is to talk about their reputation rather than their existence. It's fine to say what their reputation currently is, even though one or both might be dead.
Both the Professor (who passed away last year) and his wife are considered to be great scientists.
Another way is to avoid the verb entirely and focus instead on their accomplishments:
The Professor and his wife, both great scientists, published their latest groundbreaking research last year, a few short weeks before he died of complications from a long illness.
Unless otherwise stated, we can assume the Professor's wife is still alive.
(Edit) As DonQuiKon's comment points out, you can use the perfect tense considered to have been for past events, but this is more complicated than you might think. The perfect tense implies a relationship between two events in time, and you would not use it unless you wanted to suggest something changed.
For example, suppose you write:
The Professor and his wife are considered to have been great scientists.
This tells us very little. What has changed? Maybe they died? Maybe they are no longer considered great scientists? Without further information, we don't know.
Meanwhile the infinitive works fine for both the living and the dead:
Shakespeare is considered to be the most significant writer of English literature.
The man may have died 400 years ago, but his reputation lives on.
Both usages of are and were would be wrong without immediate clarification. And even then, it would be awkward to have some but or although added to reinterpret your phrasing.
So instead you may go with two verbs, was+is:
He was a great scientist and so is his wife.
Or rephrase this statement without a verb:
He and his wife, great scientists, ...
Very good question, this comes up a lot. I'll get to your example in a moment, but it is also worth considering this one:
Example: If a man named John died leaving a son behind,
The son would say: "I am the son of John", because the son is still alive, so he speaks of himself in the present tense.
But he might also say: "John was my father", because the subject of the sentence is his father, and he is dead so he is spoken of in the past tense.
So when you speak of people this way as individuals, it is straightforward - the living in the present, the dead in the past.
Now to your example. It should be:
He and his wife were great scientists.
This is because you have spoken of them both as a couple, and obviously you would correctly say:
He and his wife were a great couple.
.. because they are no longer a couple. Anything they did together as a couple they will no longer do together, so it becomes past tense.
However, while it is correct it does leave some ambiguity - is the living person still a scientist? If they are, you might need to qualify your statement, if it was pertinent.
Perhaps say:
He and his wife were great scientists. She still is.
or
He was a great scientist. His wife (widow?) still is.
This depends on what you are trying to say. If you want to recognize the accomplishments of the dead scientist (say, you are speaking at his funeral), then something like this would be appropriate:
Feng Zhou was an accomplished scientist. He was survived by his wife Qiu Zhou, an equally distinguished scientist.
hopefully in more detail. If the focus is on their work, follow Mari-Lou A's advice and put the emphasis on the living:
Lilah Abbas is a great paleoastronomer and her late husband Muhammad Abbas was just as well known in botany.
You have more options if they worked together:
Carol and the late John Smith were a unique partnership in topological oceanography.
I like all of the other answers and wanted to submit another option.
He, like his wife a great scientist, did blah blah blah
This option lets you quickly describe what he did to become a great scientist while also acknowledging his wife's greatness, both while not implying the wife is dead.
