I'm going on the advice of a well-respected breeder of both budgies ('keets) and cockatiels (an Australian with many years experience with both wild and captive birds of these species): Unless you have a large aviary Just. Don't. Do. It.
I'm sure people here will come up with examples of budgies and 'tiels living together in harmony, but this is the exception, not the rule. Even in the wild, budgies harass 'tiels, sometimes fatally (to either the budgie, or more commonly, the 'tiel).
This is not to say that they cannot learn to get along, with supervision, but each species should have their own cages.