If it can't go through as an iMessage for some reason it will go through as a regular text message. This frequently happens to me when I text my husband at his workplace, which has spotty coverage. For us it's fine because we also have an unlimited texting plan.
Thanks but none of that sems to address what is going on between these two iPhones. It seems like my iPhone does not recognize the other as an iPhone until it receives a msg from that phone, then my phone behaves as it should.