I have a pet budgie but I was thinking that it was lonely so I plan on getting another one but is it okay to just put the new and old bird in one cage on the same day?

It's generally not advisable to place a new budgie in the same cage as your existing bird immediately. Budgies can be territorial, especially when they are not familiar with each other. It's best to introduce them gradually by keeping them in separate cages initially and allowing them to interact in a neutral space. Once they show signs of comfortable interaction, you can consider moving them into the same cage, ensuring there are enough resources for both birds.