As someone who has been on the loss train for about 9 months, I can say that it would be a bad idea for you and your other friend to do this at the same time. Not only is she dealing with infertility, but she just had a traumatic loss and that will make her even more sensitive than she would have been dealing with "just" infertility. Two friends telling her that they are pregnant, while she just went through a horrible experience is adding salt to the wound.
I second what ^^she said. I think either e-mail or snail mail is a good idea, depending on what your more likely form of communication is. And I would have your other BFF tell her in her own way. That is unrelated to how you should tell her. I think that giving her time to grieve for herself and feel all those negative emotions that are going to come first is a much better way of doing it. She might not react how you want/how she wants if you do it in person and then it might create embarrasment or even resentment. I would just tell her in the email that you wanted to let her digest the information in her own time and you feel badly that you have to tell her this close to her loss. People who have had losses want their losses remembered, so don't pretend like it did not happen, you can mention it.
Chances are she will honestly be happy for you. But she will be sad for herself and that will be the part that she needs to digest herself, without an audience of people wanting a positive reaction. I think you are a great friend for taking her feelings into account and wanting to be sensitive to her situation. It can be so hard to be in both your place and her place. I hope things go well and you all get to have babies together...she can be the third in the line