Your code for receiving data from the I2C looks exactly the same as the usual code for receiving Serial data.
None of the code I have for I2C, looks exactly the same as Serial, in the way that yours does. But maybe, the code I have is always for the I2C master device, I didn't write any code where the arduino acted as the slave device, so maybe that explains it. My code has a bunch of "beginTransmission" and "endTransmission" function calls in it, which I don't see in yours anywhere.
I2C can have only one master device.
If you have your two trinkets working together, then wouldn't one have to be the master and one the slave ? But when you connect them both to the arduino, then you got three devices, which is the master then ?