I had the MaterniT test drawn last week and I was 11w1d. My doc said any time between 11w and 13w was ok. They also told me that the twins would not make the test or results any different. Side note it blows my mind they can separate all that DNA out! Congrats and good luck with your testing.
I had MaterniT21 done at exactly 10 weeks with results 2 weeks later. I think they said 10 weeks was the earliest it could be done. The only issue is that if there IS an abnormalty, they can't tell you which twin has it. Same with male chromosome - they can only say there is one or two males, but can't tell you which one is male or whether both are male. (if the test comes back female, then it means they are both girls).
I had Maternit21 done at 10 weeks, got results 2 weeks later. As PP said, they don't actually separate the DNA out - they are just looking for *any* abnormal DNA, so you would need an amnio if abnormal DNA is detected. Also as PP mentioned for B/B twins and B/G twins the sex portion doesn't work (but obviously learning the sex isn't really a reason to use these tests anyways). For my B/G twins I only found out "at least one male".
Re: timing of panorama/MaterNITI/harmony??