Linked cells that mirror eachother, without circular references

Hi,
'Tis a similar problem to the
a1: =b1/1.2
b1: =a1*1.2

for complimentary formulae for vat-inclusive and exclusive-columns, which produces a circular reference

This is hard to explain but actually more straight-forward, as it's similar to the above but without the calculation

I would like EITHER a1 or b1 to have data inputted, and for the other to show what the other does (unless conflicting data is written in each, which should only happen is formula are over-written with data anyway, and i'll use conditional formatting to warn me of any such conflicts)

ie

I enter Yes in A1, and B1 shows yes
or
I enter Yes in B1, and A1 shows yes

make sense?
thanks in advance