Get both cats at the same time so they can grow up together and get used to the house together removing the factor of ownership[ by the longer serving cat. What I am about to explain is my experience, which ended up being very good but doesn't always turn out this way.
I got my male cat Sushi in August 2014, and he had the house, and me to himself for almost 7 months.
When I got my female, Peanut, they were only 2 month apart age wise, but she was very playful and wanted to be around everything, which he classified as his. It was roughly a month of nightmares...they would fight, hiss, spit, wouldn't eat together, wouldn't use the same litter tray and so on...and this was after they were introduced slowly.
Then all of a sudden, found them both cuddling together, they would rub heads together and would follow each other around and if one wasn't in the same room as the other, the other would go find them and make sure they were ok.
I was lucky with mine, but I wouldn't assume that any other cat will be OK.
Reiteration, buy both at the same time, let them have their differences but also accept the fact the home is both of theirs from the start as it MAY cause more issues moving forward if the first cat you buy, takes ownership and doesn't alleviate that as easily as my boy did.