I came across this issue a few times. I reckon the strategy is during conversation, you gauge which girl is more warm to you and you steer the conversation to the warmer girl 70-30% so as to not make it too obvious, then get her contact afterwards. My sense is telling me to get one of their numbers, not both. Otherwise, you won't get any because they might think you are too player, which can work against you.
Best way to find out is to put the hypothesis into test!