How to auto populate 2 cells on different sheets if manually typed into on either sheet.

In other words on sheet 1, I4 I have a column that I use for sales price data. On sheet 2, I4 I also have a column for sales price data. What I need is if I'm working on sheet 2 and make changes to the sales price column I need it to automatically make my changes on sheet 1. And Vice-Versa. I used a formula but, once a value was added it deleted the formula so that if changes were made then it wouldn't reflect it a second time.

I hope that makes a little sense.