Credit: George Pimentel/; Michael Buckner/Getty Images

Ain't it heavy!

Melissa Etheridge and her longtime partner, Tammy Lynn Michaels, broke up in April 2010 after nine years together, but the duo are still battling over custody of their 4-year-old twins and how to divide the family's assets.

According to legal documents obtained by TMZ, Etheridge, 50, and Michaels, 36, are fighting over custody, child support, spousal support, visitation, division of property and attorney's fees. (Using an anonymous sperm donor, Michaels gave birth to her and Etheridge's twins -- son Miller and daughter Rose -- in October 2006.)

According to the site, Etheridge and Michaels are even arguing over whether they were actually married. Michaels claims they were (which entitles her to more of the Grammy winner's assets) but Etheridge says the vows they exchanged in 2003 were nothing more than a non-binding commitment ceremony.

In July 2010, Michaels blogged about being "blindsided" when her ex went to court to formally end their partnership. "I am wondering if it's an art gentle waves no noise for awhile or maybe gentle voices and SMACK!!!!! FILE FOR DIVORCE!" she wrote. "Even though we both promised, agreed, hand shook, pinky swore no filing until after tour in the fall. Her broken promises [are] told to me by headlines."