@David Bennett - So to put it another way, the 2 remaining families are buying out the 3 leaving families using a note.
In general, this makes sense, some partners want to leave, some want to stay, the staying partners buy out the leaving partners. Here are the issues that I see:
1. Price. You propose a pro rata share of the selling price of the entire property. If both sides agree that is great. There is an argument is that the fair market value of a 60% TIC interest (or however it is held) is not the same as 60% of the FMV of the whole. There is a discount, at a minimum, of the cost to partition the property by court proceeding. That is probably balanced out by the desire of the buyers to stay in that particular home. Also, of course, they have to agree on the appraiser.
2. Note terms. What is the term of the note? What is the interest rate? Is it interest only with a balloon payment, or amortized? Will the note be secured by a deed of trust on the property? They will all have to agree on these terms. Ideally, the buying owners can obtain a 3rd party mortgage and pay out the selling tenants.
3. Property taxes. Under the structure you described, in California, 60% of the property will be reassessed for property tax purposes as there was a change of ownership. Is there another way to do it to prevent that reassessment? Ask a local attorney if he can come up with an alternative structure.
Hopefully everyone can agree on the structure you described or something else, because if not, the alternative is a partition lawsuit which will probably result in less value for everyone.
I would like to hear how this turns out!