@bko in this post:
[solved] Particle Photon not receiving i2c transmission
You have a very simple I2C Photon to Photon setup. I know the connections are:
// Photon I2C setup
// GND master to GND slave
// D0 (SDA = data) on master to D0 on slave
// D1 (SCL = clock) on Master to D1 on slave
but I am not sure about pullups when both Master and Slave are Photons both pullup to 5V or can both be pulled up to 3V3?. Any suggestions?