My advice would be the exact opposite of Stephanie`s... Getting a second bird is, in most cases, GREAT for your bird. They provide a level of companionship that we can`t, even if we ARE home 24/7. In most cases birds with birdie buddies are much happier, in my opinion, then birds in one-bird households. Some older birds that were hand-raised and always lived in a single-bird environment make up the sole exception to this rule, but even they can benefit from having a friend across the room to watch and interact with from a distance.
Even if you get a bird of another species Frank will still benefit, even though they would have to be housed seperately. But I think another cockatiel would be best.
If you`re really worried abut the sex you could adopt an older cockatiel? There are usually plenty in need of a good home. But even if you end up with a female that doesn`t necessarily means that they will breed. And if they do, it doesn`t mean that you have to allow the eggs to hatch. I have a pair of adult cockatiels (a male and a female) that share a cage, and there`s yet to be a single egg. They`ve mated a few times, but no eggs have resulted.
There are many methods to lessen hormones and breeding behaviors. Rearrange the cage regularly, move it to a different location in the house from time to time, restrict all access to dark, enclosed, `nest like` spaces, and even limiting the hours of light your birds get daily can help control hormones. If eggs are layed they can be replaced with dummy eggs or frozen for 24 hours before being marked and placed back where you found them.
As long as you continue to interact with Frank regularly he will remain tame. His relationship with you could very well change a little, though. He`ll no longer be as dependent upon you so you might see a decrease in behaviors we think of as `cute` such as: pacing back and forth to be picked up the moment you come home, and wanting to spend ALL of his time with you.
I`ve tamed a number of cockatiels kept in groups or pairs (including the two I own right now), and if it`s possible to tame a pair of completely untame birds, there`s no reason you won`t be able to keep an already tame bird tame. When I introduced my cockatiel Sana in college to his new friend Cricket he was delighted to have a friend, but remained as tame as he was when he was an only bird. When I introduced him to my foster-bird, Bikko, he was ecstatic! He and Cricket always got along, but he fell madly in love with his new friend Bikko! And yet Sana remained as tame as ever, and Bikko grew tamer by the day. Sana was a much happier bird with his friends in the house to keep him company while I was in classes.
Goodluck! Just remember to quarantine your new bird. The only situation where I would feel relatively safe skipping quarantine is if you adopt a bird that`s been an only bird for years. And even then only AFTER the new bird had passed a health check from an avian vet.