do I need to grind the concrete floor before applying and would I need to apply two coats?

Floor needs to be clean of other sealers and dry,  but you don't need to grind them.   2 coats is not mandatory but I would recommend it for a longer life.

Would this be a good kennel floor? Would urine affect the coating ?

I would suggest a penetrating sealer such as Ghostshield Siloxa-Tek 8510

This will hold up extemely well to urine and cleaning.

What would happen if you applied a second coat, immediately after the first. I am having white show through in many places and it looks patchy.

The first coat wouldn't cure, as the second coat cured first, not allowing the base to clear

how much space dose 5 gal covers ?

Coverage is up to 300 square feet per gallon, so 5 gallons would cover up to 1500 square feet.

How do i apply this product?

Application Instructions