What if a married woman has got strong sexual desire that one husband can not fulfil? Is she allowed to marry two men to avoid haram relationships.
No, it's forbidden in the Qur'an:
And [also prohibited to you are all] married women except those your right hands possess. [This is] the decree of Allah upon you. ... -- Qur'an 4:24
See also Islam Awakened for further translations.
Some women have strong sexual desires, but this is not listed as an exception. Her husband should be making reasonable efforts to ensure her sexual satisfaction:
In fact her right to sexual intercourse is given priority over other rights as the main objective behind marriage is to satisfy the sexual desire of both spouses. And if the husband satisfies his wife's sexual desire he is rewarded for that if he had the proper intention before indulging in sex with her. Muslim reported that the Prophet (Sallallahu Alaihi wa Sallam) said: There is a Sadaqa in man's sexual intercourse. [ Muslim ] -- IslamWeb (hadith reference: sunnah.com)
Probably the best she can do is communicate and work together with her husband in order to become more fulfilled in the bedroom. Seeking "workarounds", like finding another spouse, is not conducive of a healthy marriage.