[michaelaye]

Actually, I don't understand why it would be such a hack/effort to have 2 projects in one file but in different group task? What is the difference between 2 projects and 2 group tasks that both can be executed simultaneously?
You can multi-select all the tasks in one group and change with one action the percentage a resource works on these.
Then you do the same for the other group task and should you put some tasks to higher priority than 100-(the other one), the levelling would even automatically shift the dates so that your resource is not working more than 100% (even so you COULD do that if you wanted it).
Additionally you can set different levelling priorities on different tasks and/or groups, so you have very detailed control if you put both in one file, doesn't seem so bad to me?
(I don't have budget experience, so that might be difficult to separate)