One or Two Placentas?!

I am currently 22 weeks pregnant with what we were told by the ultrasound tech ....identical girls!   However, my OB is convinced there may be a second placenta and the two have just fused together making it impossible for the tech to see?  I am headed to see a fetal specialist this next week but I a curious if you knew immediately about your twin/multiples placentas!? :)