If a property is equally owned by two partners, its Zakat is to be paid as a whole

وَقَالَ طَاوُسٌ وَعَطَاءٌ إِذَا عَلِمَ الْخَلِيطَانِ أَمْوَالَهُمَا فَلاَ يُجْمَعُ مَالُهُمَا. وَقَالَ سُفْيَانُ لاَ يَجِبُ حَتَّى يَتِمَّ لِهَذَا أَرْبَعُونَ شَاةً، وَلِهَذَا أَرْبَعُونَ شَاةً.Tawus and ‘Ata’ say that if two partners know their shares separately, their property will not be collected together. And Sufyan says that Zakat will not be due till one partner has forty sheep and the other partner also has the same number of sheep. ​​