OPTO TRIAC series gate resistor

In this diagram

Designer use two 100Ω resistor. Can anyone tell my why he used two resistors instead of one?
Some kind of protection against line-to-line breakdown voltage?
The line voltage is equal to 230Vrms

Two possible reasons: guard against voltage breakdown (unlikely in this circuit) or spread the power dissipation between two resistors.

It could also be a PC layout issue. He had to jump a trace and didn't want to use a jumper. Assembly costs depend on the number of different components, hence two 100 ohm resistors.