How do you partition land between coparcenaries

3rd one can file suit for partition in the court of his jurisdiction but depends that why 3rd party is not in possession & whats about 4th party/sibling ??

Considering that the property is undivided and two siblings want to sell or split the land, you can get the land's evaluation done from a valuer and accordingly divide the land equally amongst yourself. in the alternative you can also approach the court of law for partition of the land by metes and bounds.The land needs to be divided equally and each persons shared defined before it is sold or split.