collectingToBoth
This one is actually both evocative of what the method does, and in the spirit of the existing naming conventions (collectingAndThen.)
An n-ary version could just be called `collectingTo`, where it is passed a varargs of Collector. Could we get away with collectingTo for a binary version as well? The existence of the "combiner" function might make that a stretch, but I prefer `collectingTo` to `collectingToBoth`.
(I still like `distributing` too.)
