Best way to chat between two motorcycles?

One of these?

http://www.hein-gericke.co.uk/helme/kommunikation/midland-bike-to-bike-extra.html

Look here for more stuff

http://www.hein-gericke.co.uk/helme/kommunikation.html

An app doesn't seem like a practical solution. Consider what happens when you don't have data coverage. It happens even along major interstates. I was on a road trip and there was a Verizon data outage along a considerable portion of I75 in Kentucky. FRS radios were meant for this sort of use.

Sena headsets, such as SMH10. They operate over bluetooth, and have 4-way intercom up to 900 meters. It does not need a phone for the intercom feature, but will pair to a phone to get music, gps voice instructions, and incoming calls if you so desire.

Page 9 of the smh10 pdf users guide provides instructions to pair headsets directly to one another.

If you end up with an SMH10, be sure to plug in the connector hard, there is not a positive click you just have to watch it slip in. Not fully inserted it will come loose at windspeed.

The detachable mic is useful when you are riding alone and don't want the bulk.