This means that a problem with the splice one one side of the door can affect the sensor on the other side.  Lots of people see a sensor with no lights on and assume it has either gone bad or there is a problem with the splice on THAT side. It could just as easily be the splice on the OTHER side.

Always start troubleshooting this configuration on the side with the wire that leads back to the opener.  This will be the side where two pairs of wire AND the wire connected to the safety sensor are spliced together.  In the case of Genie/Overhead Door safety sensors, there may be two wire pairs the are spliced together and then connected to the terminals on the sensor (Genie/OD sensors are unique in that there are wiring terminals on each sensor to connect to directly instead of being hardwired like the LiftMaster sensor pictured above).  The far side sensor will not be live until the pre-wires are joined with each other, and are receiving power via the wire leading back to the opener.

This is where the major flaw with older LiftMaster/Chamberlain/Craftsman safety sensors really matters. It is best to make the sending safety sensor the closest one to the opener (as in the picture above). I have a detailed explanation that will be worth the read if you are struggling to get your LiftMaster/Chamberlain/Craftsman safety sensors to light up.

Back at the opener motor, you will only find one wire for both sensors because they are already paired (see pic above).