You can then use a single resizing die for 9mm & .380
A .380 is a straight walled case.
A 9mm is a tapered wall case.
Using the same sizing die means that one of them is out of spec from the second you start reloading. You can use the same seating and crimping die for either one on both calibers.
I recommend getting a separate set of dies for each caliber, at least at some point. I started off using the same set of dies for .38 and .357, which meant I had to set them up each time I switched from one caliber to the other. Not a huge deal, just got to be aggravating. So long as you are careful and ensure you are within spec, it's perfectly fine. Just not my personal preference.