Well we also had same issue a year back when we were using two instant geysers with a common pipe coming to them from the main water tank , actually it happens usually when you turn on the tap which is connected to the second geyser in series , so that causes water in the first geyser to move a bit from its position and it gets ignited , the temporary solution is to either use one geyser and put the other one a side it or use a valve or something like that our plumber installed in the course after first geyser as a result it got solved but that's temporary coz don't know why but sometimes it used to start malfunctioning often , then after fighting 3 years with these instant geysers we gave up buying the standard geyser .