Two AC lamps on an SSR board, on an Arduino Uno

// Two AC lamps on a two-channel solid-state relay board (Omron G3MB-202P).
//
//   board DC+ -> 5V    DC- -> GND    CH1 -> pin 7    CH2 -> pin 8
//   each relay's two screws in series with a lamp across the AC supply
//
// The board is "low level trigger": a channel is on while its pin is LOW.
// A zero-cross SSR switches AC only, and turns on and off near the zero of
// the mains, so a lamp starts on a whole half cycle. Here the supply is 12 V
// AC, so the lamps are 12 V ones.

const uint8_t CH1 = 7, CH2 = 8;

void setup() {
  // HIGH first, so neither lamp flashes on at power-up.
  digitalWrite(CH1, HIGH);
  digitalWrite(CH2, HIGH);
  pinMode(CH1, OUTPUT);
  pinMode(CH2, OUTPUT);
}

void loop() {
  digitalWrite(CH1, LOW);
  delay(1000);
  digitalWrite(CH1, HIGH);
  digitalWrite(CH2, LOW);
  delay(1000);
  digitalWrite(CH2, HIGH);
}