Am I obligated to pay Zakat if the price of a piece of land offered for sale does not reach the Nisab for Zakat, and other money has passed a Hawl (lunar year) but also does not reach the Nisab for Zakat, but their combined total does reach the Nisab for Zakat?

If the land was purchased for building and not for trade, there is no zakat on it. The mere intention to sell it out of need does not make it a trade commodity. However, if it was bought for trade, then assess its value when the zakat year is due, based on its price, then add its value or price to your other money and pay zakat on the total.