reuse of compute id

dear all
I want to set the temperature of atoms using only y and z degrees of freedom and use the x component to set the temperature to 300k. how can I do this

this is what I had written. is thiscoorect?
fix 12 upreservoir langevin 300 300 200 398128
compute mytemp upreservoir temp/partial 0 1 1
fix_modify 12 temp mytemp

that's the right idea. The x component will not be used at
all to compute the temp or thermostat (not exactly what
you said in your email)