Im working on a Excel sheet where I have to connect two formulas as follows:

A1 and B1 have together a value of 20%. if A1 is 12%, B1 should
automatically shows 8% and vice versa. I tried it with A1: =0.2-B1; B1:
=0.2-A1 but its not working properly. Could someone please help me?

Thanks in advance!