SPI code to daisy chain HC595 chips

Hello,
I have daisy chained two HC959 ICs together and wired them up to 16 LEDs. The code below produces a Nightriders effect with the LEDs running up and down. Everything is working well but I must be doing something wrong as the LED 8 above or below the lit one very quickly flashes on too. This would be coming from the other IC so I think the issuebut not sure how to fix it?

SPI1.setup({ mosi:B5, sck:B3 });
var i = 2,
      dir = true;
function loop() {
  
  SPI1.send(i>>8,B8);
  SPI1.send(i,B8);
  
  if(dir) {
      i = i<<1;
  }
  else {
      i= i>>1;
  }
  
  // Change direction
  if(i >= 25536 || i <= 2) {
      dir = !dir;
  }
}
setInterval(loop, 60);