I tried it in two ways. I can see that script changes their position but they somehow lose the syncer instantly and become immortal again. 
Here are the ways I tried: 
addEvent( "456", true ) 
function updatePosition(ped) 
if not getElementSyncer(ped) then 
local x, y, z = getElementPosition(ped) 
setElementPosition(ped, x+2, y+2, z+2) 
end 
end 
addEventHandler ( "456", getRootElement(), updatePosition ) 
  
  
addEvent( "123", true ) 
function updateSyncer(ped, target) 
setElementSyncer ( ped, target ) 
end 
addEventHandler ( "123", getRootElement(), updateSyncer )