Is zakat obligatory on two plots of land whose owner intends to sell them when their value increases, or to sell one of them and develop the other with its price? And what is the amount of zakat due on them in this case?

Zakat (definition) is obligatory on lands intended for trade. As for land prepared for construction, there is no zakat due on it. If one intends to sell the land when its price increases, zakat becomes obligatory on it. However, if the intention is hesitant, zakat is not obligatory. Instead, whatever is sold from it will be subject to a new hawl (lunar year), and its zakat, one-quarter of a tenth (2.5%), is to be paid if its value reaches the nisab (definition).