When our apps were written, the idea is that you would use the app throughout one pregnancy, and then start from the beginning for any future pregnancies. There is no way to see both old and new data at the same time. Our developers are looking into the best way to add such a feature. In the meantime, you do have the ability to reference past pregnancies using the backup and restore feature.
You can use BabyBump's backup feature (located under Settings) to save your first pregnancy, reset your data, then start a new backup file for your second pregnancy (and any other future pregnancies). Just name them uniquely and you can save one, open the other, and vice-versa. Just make sure you save before restoring another backup.