Well, "It happens." seems to be the most accurate thing to say about twins :P Really,I don't think there's anything a player can do to get twins. Except for,of course,breeding his/her mares :) From the technical side I think it's also important to have one spot in the Incubator left for a possible twin. And then just breeding and waiting. But it does happen quite often,as you can see in this topic, so don't loose hope when you haven't got twins for a long time! :)
Mine were born today, here they are:
The Earth's color made me sure that they'll stay in my herd - she looks like Christmas,hihi ^^