I have 2 sister cats age 2.5 years old, they were spayed very young. They always fight for who gets to be on the very top of our cat condo/cat post and then after fighting for left then 10 seconds start licking each other. I don’t get it. Should I just buy another cat post for them?
Sometimes they do hiss at each other while smacking each other in the face. But they only hiss maybe once out of every 10 times they fight over it.

Are they actually fighting, or are they just playing with each other? Friendly play can look violent, but it is perfectly normal for cats raised together to wrestle and bite each other in good fun. Normally, if cats are actually fighting in earnest they will howl and hiss at each other, and generally make lots of noise. They will also injure each other. The fact that they are breaking off midway to groom one another implies to me that your cats are not really fighting. This is typical play behavior.

Either way, high ground of any sort is considered prime territory by cats, so it is normal for them to fight or play-fight over who gets to occupy this space. Cats who are friends with each other will usually work out their territorial claims in a playful way. I’m sure your cats would enjoy having a second cat post, but I don’t think you need to be concerned about deterring this behavior.