You could remove [PARTICLESTREAM_1] from AtlasICBM.ini in multistage...because multistage already have default particlestram...only need to leave texture.
[TEXTURE_LIST]
TEX_1=Exhaust2
TEX_2=Contrail2
...
...
...
eng_diameter=0.4
ENG_TEX=Exhaust2
ENG_PSTREAM1=exhaust
ENG_PSTREAM2=contrail
[TEXTURE_LIST]
TEX_1=Exhaust2
TEX_2=Contrail2
...
...
...
eng_diameter=0.4
ENG_TEX=Exhaust2
ENG_PSTREAM1=exhaust
ENG_PSTREAM2=contrail