Use the built in $RANDOM function and arrays.
DF_BCH_CB="file1.sdf"
SDF_BCH_CW="file2.sdf"
SDF_BCH_RCB="file3.sdf"
SDF_BCH_RCW="file4.sdf"
SDF_TT="file5.sdf"
ARRAY=($DF_BCH_CB $SDF_BCH_CW $SDF_BCH_RCB $SDF_BCH_RCW $SDF_TT)
INDEX=(0 1 2 3 4)
N1=$((RANDOM % 5))
SDFFILE_MIN=${ARRAY[$N1]}
N2=$((RANDOM % 4))
if [ "$N2" = "$N1" ] ; then
N2=$N1+1
fi
SDFFILE_MAX=${ARRAY[$N2]}
echo $SDFFILE_MIN
echo $SDFFILE_MAX
This gets two different strings for SDFFILE_MIN and SDFFILE_MAX. If they don't have to be different, remove the if statement in the middle.