Is there a way to add two calculated fields together that are from two different object types?

Yep - as long as both fields are number fields and you are doing the combined calculation on the "lower" object type. (So in the case of Project and Task, you would have to do the calculation on the task. The reason is that you can reference in calculations, but you cannot use collections).

Not sure what fields you are using. But let's say had Budget Left Over fields on both the object types. And I want to see what is the value left on the task level. I could do...

Project.Budget Left Over-Budget Left Over

(Not the best example, I know, but I couldn't think of a better one #FridayBrain 😜 )

Hope that helps