[db48x]

Unless you need two such programs on the same machine, in which case they need to coordinate somehow. Perhaps by having a third service which hands out the ids as needed.

[JdeBP]

... or by having a shared-memory account database in a tmpfs at /run/systemd/dynamic-uid/ . (-:

That idea, by the way, was invented more fully and more generally about a decade and a half ago by Daniel Rench, who named it "userdir".