According to IL Publication 108, page 2:

If you and your spouse each have a principal residence, or if you had two principal residences during the tax year due to the sale of your home, you may claim the tax paid on both residences when figuring this credit.

Therefore, if you have owned and paid property tax on two primary residences in IL, you can enter the other property number in line 4d of Schedule ICR since your adjoining lot on your residence is already entered in line 4c.