Times like these a "Savoy" clause s pretty helpful. Principle is pretty basic, if party A wishes to buy party B out then A nominates the price they are willing pay. Party B then has 30 days (or whatever timeframe is agreed) to match that offer and buy Party A out or if Party B chooses not to do so then Party B must sell to Party A at the nominated price.
No perfect, particularly if there is a monetary imbalanc ebetween the partners, but a reasonable way of ensuring both parties think the price is fair.
Equally if you keep the partnership going I'd be asking for costs associated with ferrying.