Allah S.W.T can do everything, but He will not do something that does not befit Him, or goes against His Attributes. It is like asking if Allah can have a son. Or even more absurdly, can Allah be God and not be God?
The question is essentially invalid at first place. A God by definition is the creator of everything. So if something is created, then it can't be God. And if a "god" does something which goes against his attributes, then it ceases to be "god". So, the question "Can Allah make another God?" has a contradiction, hence meaningless.
It is certainly a kufr thought to think of, but kufr doesn't apply to the person for just having the mere thought...
It is narrated on the authority of Abu Huraira that some people from amongst the Companions of the Apostle (may peace be upon him) came to
him and said: Verily we perceive in our minds that which every one of
us considers it too grave to express. He (the Holy Prophet) said: Do
you really perceive it? They said: Yes. Upon this he remarked: That is
the faith manifest.
Book: The Book of Faith - كتاب الإيمان,
Global Id: 10355 (0) English reference: Book 1, Hadith 239
This indicates that if someone gets a kufr thought but resists it and drives it way, then it is a sign of faith. It is only an act of kufr if someone actually believes in those thoughts. The best thing to do is to not continue thinking about these thoughts when they come up, especially that they are invalid.