I made a vow, but I cannot remember what it was. My husband went through a difficult experience and made a vow to Allaah, the Exalted, that he would slaughter a sheep every month if He relieved him of his calamity. I cannot remember whether I had also vowed to slaughter a sheep every other month, given that I work and have a stable monthly income.
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you doubt whether you had vowed to exchange turns with your husband in slaughtering an animal every month, (each of you offer an animal sacrifice every other month), then you should do so as it is more prudent for you in order to be on the safe side in terms of clearing you of your liability. This is because one's liability cannot be cleared except with certainty (i.e. incontestable evidence and not mere doubts). Therefore, you should discuss this matter with your husband.
We understand from your question that your husband actually took a vow to slaughter a sheep every month, but you doubt whether you vowed to slaughter a sheep every other month by taking turns with him. If he agreed to slaughter a sheep every month, then it is more prudent to intend to slaughter the sheep on one of the two months on behalf of the one of you who is obliged to offer it, either you or him. There is no problem if you financially help him with whatever you can afford. If he refused to offer a sheep every month, then it is more prudent for you to slaughter one sheep every other month so as to make sure that you fulfill your vow.
Allaah Knows best.
You can search for fatwa through many choices