Praise be to Allah.
There is nothing wrong with your wife taking natural or pharmaceutical fertility drugs in the hope of conceiving twins, on condition that this medicine is free of any harm, because the Prophet (blessings and peace of Allah be upon him) said: “There should be neither harm nor reciprocating harm). Narrated by Ahmad and Ibn Maajah (2341); classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
But you should consult a trustworthy doctor to see whether this is a good idea in principle, and ask whether there will be any harm caused to the wife after that or not, then consult him about the type of fertility drug.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: What is the ruling on taking fertility drugs for the purpose of getting pregnant, noting that I have been married for a long time and I do not have children?
He (may Allah have mercy on him) said: This matter requires consultation with a doctor. If he says that taking fertility drugs in order to get pregnant is not harmful, then you should use them to help you to get pregnant, because the Prophet (blessings and peace of Allah be upon him) said: “Marry women who are loving and fertile, for I will be proud of your great numbers.” (Fataawa Noor ‘ala ad-Darb).
See also the answer to question no. 11906 .
And Allah knows best.