how do i send and receive signal using two arduino uno(ATMEGA 328P-PU) with simulink

i have two separate simulink files, one for sending signal and one for receiving sugnal. 
the reason i have two separate files is b\c i have to difine the Ports separatly. 
the problem is when i run the first files which is the file sending the signal works ok, 
but then when i run then secound file which is receving i get this ERROR

"Error occurred while executing External Mode MEX-file 'ext_comm': Failed to connect to the target. A time-out occurred while waiting for the connection response from the target. Possible reasons for the time-out: a) The target is not switched on. b) The target is not connected to your host machine. c) The application for the model is not running on the target. You might have clicked the Stop button. If the Run button is not dimmed, click it. Otherwise, click the Build button, which downloads and runs your application on the target."