Singular or plural verb form with 'What Ø the cause, progress and result of your actions?' [duplicate]

I am having trouble with the following sentence:

"The complexity and diversity of the new platforms is worrying us."

If "complexity" and "diversity" constitute two subjects, then the verb should presumably be "are"; however, the sentence feels more natural with "is". Is this correct? Is there a linguistic term for this occurrence?

The following questions address the general topic of compound subjects, but not specific cases like this:

[Singular] Is/Are [Plural]?
Singular or plural following a list

And the following question seems to have somewhat inconclusive answers:

Verb agreement in “Where is the Messiah and his Kingdom?”

So this question is not really a duplicate of these three.

Solution 1:

Yes, it is just fine. Compare:

• “My lord and husband is come home.”
• “Peanut butter and jelly is the best sandwich of all.”
• “Simon and Garfunkel sounds really mellow right now.”
• “Tide and time waits for no man.”
• “Our President and CEO is Joe Schmoe.”
• “Running and jumping is all I want to do today.”
• “The master and commander has not yet returned.”
• “Yesterday is gone; today is fleeting as we speak; tomorrow and tomorrow and tomorrow is all that remains to us.”

It happens when the subject is thought of as one thing. Sometimes it is two things that are so fused as to be thought of together. Sometimes it is two names for the same thing.

Do not let that little and make you think that a plural verb must necessarily follow. It does not have to, and sometimes should not.

Solution 2:

The number of the verb depends upon whether the worry is caused by the diversity, the complexity, or their combination.

If you could cope with either the complexity of the situation or the diversity alone but in combination they cause worry, then the singular subject requires the verb "is".

On the hand if the complexity alone would cause worry, then there are (at least) two causes that demand a plural verb "are".