Is he sinning if he keeps one of his wives with him and sends the other back to his homeland?

No File Exists
Check below answers in case you are looking for other related questions:
Ruling on plural marriage and conditions thereof
He is away from his country and his wife and he wants to take a second wife

No File Exists
Related Answers:
If his wife does not agree to him being away for more than six months
Ruling on kissing and embracing one wife in front of her co-wives!
Polygamy and respecting the wife’s feelings
Ruling on plural marriage and conditions thereof
He is away from his country and his wife and he wants to take a second wife

Recommended answers for you:
The date for ‘Eid al-Fitr and Eid al-Adha is based on sighting of the new moon
Khula’: definition and how it is done
Praying at graves and the conditions of intercession
Ruling on committing suicide because of depression
Definition of the blood which invalidates the fast