Weight and size of placenta (twins) 20 weeks?

Ultrasound tech said on eplacenta (ind twins) doctor said Frat, the placenta to large to be one. Fused or together placenta. There is also a thin membrane between them. Twin A boy, twin B tech thinks is boy,not sure but she thinks ind twins so may be boy. Dont want to get my hope u thinking could be frat and twin B really might be girl. So what the wieght and size of one placenta for 20 week twins? Or any web site