grammar - Do "direct object を" and "directional を" occupy the same slot?

A Dictionary of Basic Japanese Grammar (ADoBJG) lists more than one を particle. In particular:

• On page 347, it lists o¹, a particle which marks a direct object.


• On page 349, it lists o², a particle which indicates a space in / on / across / through / along which s.o. or s.t. moves.

The second particle, which I'll call directional を, appears to be used quite differently from the direct object を. According to ADoBJG, the directional を is used only with verbs of motion, which I think are generally intransitive. For example, directional を appears in the phrase 空を飛ぶ, combining the intransitive verb of motion 飛ぶ with an を-marked noun.

Since it's listed as a separate particle in ADoBJG, and since it behaves so differently, and since it appears in different contexts from the other を, I've always thought of directional を as a separate particle which happens to be written and pronounced the same way. However, in 国語辞典s such as 大辞林 and 大辞泉, both usages are listed under the same entry. So I'm wondering how independent the two really are.

It seems to me that, because directional を appears with intransitive verbs, and because direct object を appears with transitive verbs, the two occupy separate syntactic slots. That makes me wonder if it's possible to take a construction such as 空を飛ぶ, which uses the directional を, and turn it into a transitive construction which also takes the direct object を, such as ◯◯を空を飛ばす.

My thought is this: if they don't really occupy different slots, then this is ungrammatical because of the double-o constraint, which says that a single verb phrase may have no more than one を-marked noun phrase. But if they do occupy different slots, and they're really different particles, then you should be able to create a single verb phrase containing both.

I'm not sure my ◯◯を空を飛ばす example is very good. (It's supposed to mean "send ◯◯ flying through the air".) But if it's possible to create a grammatical example combining both directional を and direct object を, I would like to know, regardless of whether my particular example is very good.

Is it possible to create such a construction?

Answer

I posed this question to a native Japanese speaker. Her response was that even though to an English speaker the を in

ボールを投げる

and the を in

道を歩く

may seem different, to a Japanese speaker they are exactly the same. In both cases, を marks the direct object, not in some vague grammatical sense of the term, but in the very tangible sense that the object is directly acted upon by the subject.

For a Japanese speaker, the 道 in

道を歩く

is not a piece of background scenery that the subject of the sentence simply floats past on their way from point A to point B; rather, it is tread on, pushed, even kicked, by the subject's feet.

For an English speaker, this is a rather alien concept, but for a Japanese speaker, to walk along a road is to act on the road. を serves one function: marking the direct object.

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Update: Searching the web, I find results that contradict my original answer. 「投げる」 is a transitive verb (他動詞) while 「歩く」 is intransitive (自動詞), even when used with 「を」 (as an aside, this is unlike the verb "walk" in English, which can be either transitive or intransitive). According to these sources, when 「を」 is used with a 自動詞, it does not mark the direct object, as a 自動詞 cannot take a direct object.

I found an entire 100-page paper on the subject of 「を+自動詞」 which looks fascinating, but I have yet to read. Today I also intend to ask the opinion of two more native speakers, both Japanese teachers.

Finally, the Japanese wiktionary page on を directly answers your question:

用法
一つの単文の中で、をは一つしか使われない。従って目的語と経路のをは共存できない。
馬（うま）を通（とお）す。
門（もん）を通す。
*馬を門を通す。 (誤り)