I am Emsumm's DH, she wanted me to reply to this directly. We have 4 month old twin girls who are thriving thus far (at least I think they are).
To help your wife be successful breastfeeding, you should take on all the dirty work, changing diapers before/after feedings and changing their outfits after they spit up all over themselves. It is also important that you try to occupy whoever isn't eating (if your wife isn't tandem nursing) whether they are crying and you are trying to calm them, or playing with them to keep them in a good mood. The hardest thing for your wife will be being occupied with one baby while the other is upset because she can't just put down a happy baby to help a crying baby because you are more likely to end up with two crying babies than two happy babies if that has to be the case (and one stressed out momma).
The most important thing about being a dad of twins is to support your DW because there will be times when she just needs to vent to someone who can actually understand what she's saying. She may not have a lot of time with her friends to talk about any issues she is going through, and she will need you to be encouraging and supportive.
Feel free to pm my wife if you want to discuss anything else.