Yes, it's possible to have two periods a month. Most women have one a month, but some women are really irregular. They might have two periods a month, while other women might have one period every three months. It varies from woman to woman.

As far as the rhythm method goes, I certainly would not suggest using it unless you two want a baby. It's not a very reliable method even for women who do have a regular period. If her period is irregular, it will be even more difficult. I would suggest getting a trusted method of birth control. There are many options that are very effective.