In this model, two transmitters send messages with duration 1.0 periodically. The messages from Transmitter1 collide with every second message from Transmitter0, causing both messages to be lost.
If you change the SNRThresholdInDB parameter of the Receiver to 3 (3dB is a factor of two in power), then the messages from Transmitter0 will be successfully received despite the collision.
If you then move Transmitter1 closer to the Receiver, then every second message from Transmitter0 will again be lost.