The business has reached its peak. The remaining person will find it psychologically impossible to create new value working solo, with the knowledge that 40% of it will be taxed by the ex business partner. No investor will put new money into an enterprise where 40% of the cap table is not involved. You are not splitting future growth, you are now dividing present assets as this won’t be a going concern much longer.

Your options are either reduce your stake to keep the business viable for remaining parties, or sell it completely for a fair price. To uncover the hidden fair price, one person decides the share price, and the second decides if they will sell their stake or buy out the first.