My personal view on it would be that the acceptability of this depends on how much overlap there is between the projects, and how deliberate you were at the outset in making things a single project.
If you have one class which is asking for a project which does A and another which is asking for one which does B, and you come up with a project which does both A and B, then that's likely okay. However, if they're both asking for slightly different variants of A, then it's less likely that a single project which does just A will be accepted.
For example, say you're in a Computer Science class which asks you to write a non-trivial program using multithreading and in a Biology class which is asking you to do a project on DNA sequence alignment. I think that most professors will think it actually a bit clever and pedagogically useful for you to write a computer program which does DNA sequence alignment in a multithreaded fashion to satisfy both projects. Similarly, even if it's two CS classes it can work if the courses are different enough (e.g. a compiler course and a GUI course).
A large part of this is how much "reuse" between the different project requirements there is. For example, in the multithreaded DNA alignment case, the part of the project the CS professor is interested in (multithreading) is disjoint from the part the biology professor is (DNA alignment). You're not really being "double graded", as even though project is the same, the core part being graded is different. -- It's less that you have one single project that you're submitting twice, and more that you are fusing the two distinct projects into one cohesive whole.
The other aspect is the deliberateness of combining. The point of projects isn't make work, it's to provide a demonstration that you have learned and are able to apply the course materials. There's thus an implicit requirement that the project should reflect and incorporate the course materials. To do so effectively, you need to have the class materials in mind while doing the project. This is one reason why reusing projects from previous semesters is frowned upon - you couldn't have consciously incorporated materials from the course in a project made before you took the course. That also potentially applies to projects made in the same semester. Did you deliberately set out to fulfill that specific project's requirements (incorporating and keeping in mind the course materials), or did you just happen to notice after-the-fact that the project for course 0 could be made to work for course 2?
That said, there can be a big difference in opinion between different professors. You definitely don't want to attempt to "slip one by" them. You should be upfront about the fact that you're combining projects. In certain cases, an attempt to deceive (even via omission) can be more ethically dishonest than the project combining itself.