Is it permissible for a woman to marry more than one man, and if her husband refuses, is it permissible for her to marry secretly, on the pretext that Allah did not mention its prohibition in the Qur’an?

Marrying a married woman is forbidden. This is a matter known by necessity in Islamic law, and whoever denies it after knowing it has apostatized from Islam. The Quran explicitly states this ruling in His saying, "And [forbidden are] married women." A woman is not permitted to marry before her 'iddah (waiting period) has expired. If she marries before the 'iddah expires, knowing this, then she is an adulteress, as is the one who marries her. Both are subject to the prescribed punishment if this is proven before the ruler.