CDO

Hi
I am having 10 files of present and future of different models in a folder in with each file is starting with (pres) for present and fut(future). AS I want to read them different with there names and then I want to subtract it with there respective models present and future file. Can it be possible in cdo?
#!/bin/ksh

infile1=fut*.nc
infile=pres*.nc

for t in $infile1
do
for f in $infile
do

cdo sub ${t} ${f} sub_${t}${f}
done
done
exit

Error-
cdo sub: Processed 20712960 values from 2 variables over 744 timesteps ( 1.16s )

cdo sub (Abort): Grid size of the input parameters do not match!

cdo sub (Abort): Grid size of the input parameters do not match!
cdo sub: Processed 6094848 values from 2 variables over 744 timesteps ( 0.27s )

As it is giving me the right output of 2 files of present and future for two models, but it again using the present file of one model and future of other model and I am getting the error of grid size. SO how can i remove it , so that it subtract the correct future file with the correct model present file, As at the end I have to take ensmean(ensemebel mean ) of all model.
Waiting for a positive reply.