Its an interesting question. What exactly makes them coupled? Are you expecting to reuse this combination in the future? There are a few ways to do it:

Call validations from the change

You could combine it by calling the validations in the change. If you have a reusable validation that you want wrapped into a bigger change.

with :ok <- Validation.validate(changeset, ...),
        :ok <- OtherValidation.validate(changeset, ...) do

end

Macro it

You could define a macro that includes both:

defmacro check_and_validate(key, param) do
  quote do
    check_something_aboute(:forms, extra_param)
    do_something_to(:forms, extra_param)
  end
end

And then in the action

create :create do
  change_and_validate(:forms, extra_param)
end

Share functionality with functions

If they are coupled in terms of functionality, then you could define a module with functions in it that allow them to share this functionality.

def change(changeset, _, _) do
  SharedBehavior.foo(changeset)
end

def validate(changeset, _) do
  SharedBehavior.foo(changeset)
end

Put it all in the change

At the end of the day, its perfectly acceptable to do a “validation” inside of a change, returning an error as you mentioned. Change modules are meant to be that outlet when their simpler counterpart, validations, don’t suffice.