two oil burner piping problem

Every now and then i run into a system with two oil burners conected to under ground tank with two lines coming in feed and return and when they are both running one will go off on safety,they have been running fine for fifteen years,they have filters on each and no check valves,the only way i know how to solve this problem is to install a TIGER LOOP for each burner,can you tell me what the problem is and how to solve,both also have new one stage pumps,thanks,oilman
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