Capital gain in sale floor wise

You can invest capital gain in any bond specified u/s 54EC of income tax.
For example if cost of 1 floor after indextn come out to be 9 Lakhs and you sell the same for 17 lakh you have to invest 8 Lakhs in notified bonds u/s 54EC with in 6 months from sell took place for a locking period of 3 years.

since you are planning to sell the 2 floor in 2020 you can purchase a under construction property now which will get completed after 2yrs from now. the capital gains of one floor sold now can be deposited in capital gains deposit account for current ay and claim exemption.
when you sell your second floor after two years you can utiluze the cost of the new flat by investing the capital gains of both the floors in 2020.
Alternately you can invest the capital gains of the first floor in 54EC bonds upto maximum 50lacs for 3 years lockin period and then after selling your second floor you can invest in new property

You have an another option as per section 54 if the house is residential house and income so derived is shown as income from house property.
You can either construct or purchase a new residential house within India (purchase with in 2 years after the date of transfer and construct within 3 years after the date of transfer).

Same above example.
If capital gain comes to be 8 lakh you have to spent the same for new/ Construction of a residential house. Than entire 8lakhs will be exmented from LTCG.