Cockatiels, like budgies, are BIG time flock birds and unless you can give them at least 12 hours out of cage time each and every day, need at least one other cockatiel around them for socializing. The flock means that much to them.
When you get his companion, get that bird his own cage. Never just shove a new bird into an existing bird's cage. The cage is their territory and they could decide to fight the new bird to the death to defend it. Once the two birds know one another, then you can consider caging them together.