How can I wire two push button phones to each other so I can talk at one phone and hear the voice at the second phone as well as talk back to the first phone from the second phone? I heard that I could use a 9 volt battery as the power source. The phones are to be used in a first aid event and there will be no electricity available other then a battery.

Here's a site that shows it can be done using a 12-volt battery and a 680-ohm resistor. It also claims that a 9-volt battery would work as well, but I suggest you should test it to make sure the battery will last long enough for your event.

Using old phones as an intercom in your VW bus (or anywhere else) - Hack a Day (http://hackaday.com/2012/06/08/using-old-phones-as-an-intercom-in-your-vw-bus-or-anywhere-else/)

Here are 2 Very Good Ones,
More money, nut more features, I have good luck with Viking:
Viking Electronics Telecom & Security Products (http://www.vikingelectronics.com/products/search_cat_results.php?mycategory=11)

These are good for 2.6 miles? Kind of Handy