I'm chasing several girls. I invite one to come along with me on an outing with friends (any general hang out activity, like a party or camping) with something like "Hey, I'm going to do X with some friends, it should be a good time, you should come along". She declines for whatever reason. I ask another girl, she accepts and plans on it. Later, the first girl I contacted tells me that she will be free after all and would like to come along.

What do I do in this situation? Take both girls with me? Make up an excuse to tell the first girl why she can't come anymore?