There is a difference, depending where you make the split:
Say Project A takes 1 month to dump, Project B takes 1 month to dump, and 30 other projects take 1 day each to dump. Say you're CPU bound and have 2 CPUs, so you run 2 threads.
Method 1: Project A and Project B take 1 month to complete. Then the rest of the projects take 1/2 month to complete. Time between successive dumps is 1.5 months.
Method 2: Project A takes 1 month to complete while the 30 other projects complete. Then Project B takes 1 month to complete while the other 30 projects complete again. Time between successive dumps is 2 months for projects A and B, and 1 month for the rest of the projects.
Indeed, you can make some dumps more frequent at the expense of making others less frequent. No-one has yet explained why small dumps should be more frequent that large ones.
On the other hand, if the 30 "other projects" took 4 days to complete, the times would be 2 months by method 1, and 2 months/4 months by method 2. It all depends where you make the split, and it's not clear to me what split is the most "fair".
You've made a mistake in your maths (or, a typo) - method 2 should have one thread dumping more frequently than method 1 and the other less frequently, you have one thread the same and the other less frequent (which would be strictly worse than method 1).