A rep for Trammell confirmed that Yager gave birth to the boys earlier this week, reports E! Online.
Trammell's spokesperson declined to reveal the boys' names or provide a further update on their condition.
Yager and Trammell first began dating in 2003, when both were performing in a Broadway play.
Trammell recently predicted that it will be "tough" for True Blood to ever earn an Emmy Award.
True Blood continues with new episode 'Spellbound' on Sunday at 9pm ET on HBO.
> Denis O'Hare teases True Blood return
> True Blood Deborah Ann Woll 'excited about romance plot'
Watch a True Blood season four trailer below: