$ cat test.sh
x=`echo hello | cat 2>&1 >/dev/null`
if [ -n "$x" ]; then
  echo WRONG
else
  echo RIGHT
fi
y=`echo hello | cat >/dev/null 2>&1`
if [ -n "$y" ]; then
  echo WRONG
else
  echo RIGHT
fi

$ ksh test.sh
RIGHT
RIGHT

$ ksh -x test.sh
+ + echo hello
+ cat
+ 2>& 1 x=1> /dev/null
+ [ -n 1> /dev/null ]
+ echo WRONG
WRONG
+ + echo hello
+ cat
+ 1> /dev/null 2>& 1
y=
+ [ -n  ]
+ echo RIGHT
RIGHT