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The I-751 generally must be filed with both spouses jointly in order for the beneficiary spouse to remove conditions on their green card. Sometimes, this is not possible. There are multiple options for seeking a waiver of the joint filing requirement. If appropriate, you can seek a waiver on more than one ground. Each option has benefits and drawbacks. The waiver options include:
(1) The marriage began in good faith, but was terminated through divorce or annulment. This can only be approved when the divorce is final.
(2) The marriage began in good faith, but the US citizen or LPR spouse was physically abusive or imposed extreme cruelty. Emotional abuse can suffice here.
(3) Your spouse is deceased. This one is self explanatory.
(4) The removal of the conditional resident from the US would result in an extreme hardship. A benefit of this waiver is that proving a bona fide marriage is not an absolute requirement for approval on this ground. The extreme hardship waiver is discretionary and has a high standard to meet for approval. Here is a link to the section of the USCIS policy manual that describes how they evaluate these cases: