A simple option would be to have both tied to the second engine start, perhaps with a slight delay after the engine has actually started. As such:
<Effect Type="StartEngine1" EffectName="Ar234_Rocket_Exhaust_flame1" Location="emitter_rato_l" />
<Effect Type="StartEngine1" EffectName="Ar234_Rocket_Exhaust_flame1" Location="emitter_rato_r" />
and
<Ar234_Rocket_Exhaust_flame1 ClassName="ParticleEffect" EmitterShape="Point" EmitterRadius="0" EmitterLifetime="30" EmitterPosX="0.0" EmitterPosY="0.0" EmitterPosZ="-.15" ZBias="0.00002" EmissionAngleHoriz="180" EmissionAngleVert="90" EmissionSpread="3.5" EmissionRate="70" EmissionRateVar="15" InitialDelay="5" InitialCount="1" MinDistance="30" MaxDistance="2000" StopMethod="1" ZSort="1" UprightSprite="0" RandomStartRotation="1" RandomRotationDirection="1" Lifetime=".15" LifetimeVar="0" Speed="10" SpeedVar="0" MaxSpeed="1600" MaxSpeedVar="0.0" Accel="0" AccelVar="0.0" AccelTime="0.0" Gravity="0" Drag="0" DragDelay="0.0" InheritVelocity="1" Size=".4" SizeVar=".05" MaxSize="0" MaxSizeVar="0" GrowRate=".015" GrowRateVar="0" RotationRate="200" RotationRateVar="100" FadeInTime=".05" InitialColor="250 194 80" InitialAlpha="255" Color="254 250 192" Alpha="255" FadeOutTime=".08" FinalColor="253 251 151" FinalAlpha="0" Texture="Fireball_Additive2.DDS" BlendMode="Add"/>
You'd want to make sure the smoke effects that look like they are part of it have the same delay, just used 5 seconds as an example, could be whatever you like.
Unless you have a third engine set up in the FM for the RATOs, as it looks like you might, in which case the effect type would be StartEngine2 and no delay needed.