They can both eat the same food, but the cockatiel can handle bigger pieces than the budgie can. There are several good brands, but my cockatiels' favorite is Nutriberries. They have the same nutritional value as standard pellets, but they haven't been ground up as fine so they look like seed balls. It's usually easier to convince birds to eat Nutriberries than it is to get them to eat other pellets. The company is good about sending out free samples: https://lafeber.com/contact-lafeber-co/
Nutriberries are expensive . I’ve noticed with cockatiel nutriberries they do not use hulled safflower seed . This seed is high in fat and my tiel would just pick out those seeds and ignore the rest. But if you buy the nutriberries for budgrigars you shouldn’t have this issue.
You can feed them the same seed, and pellet as well, but some tiels will prefer pellets a little bigger, while others like small seed sized pellets.
I recommend roudybush nibbles.
I also recommend parakeet /budgie seed mixes without safflower, peanuts or sunflower for your tiel and budgie.
Lastly, do try to make 50% of the diet vegetables and/or sprouts. They don’t need to eat a lot of pellets. 10-20% of their diet is fine. The more veges they will eat the better. Dark leafy greens, broccoli, carrot have loads of vitamins and calcium also. Boiled egg can be offered once a week, fruits with no pits and non toxic can be offered once every 2-3 days. Spinach should be offered once a week, it has oxolates which can prevent absorbtion of calcium.
Budgies and Cockatiels have similar diets, so using budgie pellets should be fine. HOWEVER, cockatiels require a more fat in their diet than budgies, so make sure your supplementing their diets with fattier seeds, such as pumpkin, sunflower, millet, etc. in moderation. Another supplemental food that is a little higher in fat, usually recommended for breeding, molting, and weaning babies is miracle meal: https://www.amazon.com/Morning-Bird-MB076-Miracle-Pound/dp/B012UK5N6I
I haven't tried it yet personally, but i've heard good things about it.