if the co-owner withdraws from the deal, then the buyer can enforce specific performance of contract against him
as the last date of execution was 2.1.19, but the buyer has made payment on 20.1.19 or thereafter, it means that buyer by implication has agreed for extension of contract
now it is for the buyer to enforce the contract against the sellers or against the co-owner seller who wants to back out
the co-owner seller can back out only if there is a breach of contract by the buyer as stated in the contract
also possession of 2/3rd undivided share cannot be given unless the property is divided and partitioned by metes and bound
but if the buyer only wishes to acquire 2/3rd rights without the possession, then its totally upto him