Is it possible to do this:

let's call cell A1 "inches", and B1 "cm"

if the user enters a number in A1 (then hit tab or enter), the equivalent
length in cm will be displayed in B1

if the user enters a number in B1, the equivalent length in inches will be
displayed in A1

Obviously one cannot store the forumulas in either A1 or B1 because the
formulas would get erased when the user enters a number