If I understand what you want this can be done with an array formula**.

Something like...

X1 = set value

=STDEV(IF(A1:A10>X1,A1:A10))

=STDEV(IF(A1:A10<X1,A1:A10))

** array formulas need to be entered using the key
combination of CTRL,SHIFT,ENTER (not just ENTER).
Hold down both the CTRL key and the SHIFT key
then hit ENTER.