I can tell you one thing... getting a kitten to be Ellie's companion was the BEST decision I EVER MADE. I got Ellie in October of 2013... at the time, my boyfriend and I could only comfortably afford and handle one kitten. We adored her... but boy, she was a big handful. Constantly attacking our ankles, constantly acting out and being obnoxious, always over-bored, no matter how much we played with her. We knew she needed a friend, especially since she is very attached to us and got incredibly lonely when we left her.
So fast forward to 2014... we got Tootsie, a kitten from Ellie's same breeder, as Ellie's companion a little after Ellie's first birthday. It took about 2-3 days for Ellie to accept Tootsie, and then they instantly became best buddies. From this time on... they played perfectly together. They cuddled, played, ate, did everything together, and still do to this day. Tootsie was an angel of a kitten (still is!) because Ellie would put her in her place and tire her out with playtime... and vice versa. Ellie has been an absolute ANGEL since we brought Tootsie home. She finally isn't bored! I have not been bitten or play-attacked by a kitten since. Now my kitties are just little angels. And I mean that! I honestly can't believe Tootsie is still only 6 months old... I feel like I have two super well-behaved adult cats that are a little more playful lol.
Seriously, in the future, I will never... never adopt a kitten without adopting a second as their companion. I learned that the hard way with Ellie! Ellie was by no means a bad kitten, she is a Persian so was honestly a little more calm by nature. But she was bored and always acting out, and getting Tootsie cured that for sure, and made my relationship leaps and bounds better with Ellie because I'm not yelling "no!" at her constantly anymore for acting up! haha.
I have two litter boxes, two cat trees, and a bunch (4-6) of cardboard scratchers dispersed throughout the apartment, as well as some toys for them to play with. They are eachother's shadows, so they are always on the same cat tree, or playing together with the same toy, etc.
I will say though, I feel like it's important to try to find a kitten with a similar temperament and personality to your current cat. Tootsie ended up being an absolutely perfect match for Ellie, and they've been inseparable since. They're both very sweet, laid-back cats. Ellie is a little more fearful, whereas Tootsie is more fearless and outgoing... so they balance eachother out perfectly!
Good luck if you do get another kitty!