Garage Sensors Work Separately but Not Together?
The blinking red light on the sender would indicate that they are mis-aligned. The best way to test this is to remove them from the mounting brackets and hold them a couple of inches apart with the sensors aligned. Have somebody hit the door closer, then point them away from each other.  If they work as expected that way, you'll just have to fiddle around with the mounts until you have them correctly aligned.