Is it possible to put two adverbs one after another?

Question

In the following sentence: 本当に正直に話していいかな？ I know that 本当に is an adverb which means "really" but what about 正直に? I though that it was an adverb because of the に but I don't know if one can put two adverbs one after another just like in above sentence, but if that's not the case... then that に is acting as a particle?

Answer 1

It’s not entirely what you were asking, but an important point is that 正直に modifies 話す, while 本当に modifies いい. You can see this by the following scrambling being valid:

✔️ 正直に話して本当にいいかな？

(Though note this is a somewhat unusual word order.)

While this scrambling makes no sense at all:

✖︎ 本当に話して正直にいいかな？

This also explains why you can’t say something like ✖︎ 本当で正直に話していいかな？, because it makes 本当で正直に be a single unit, which doesn’t work because neither predicate (話す or いい) in the sentence can accept both of those modifiers semantically.

On the other hand, if we change one of the modifiers to be something which does make semantic sense to use with the same predicate, combining them into a single unit works fine, like 真面目で正直に話していいかな？.

Syntax trees

If it helps, here’s the syntax tree breakdown:

[本当に][<正直に>話して]いいかな？
_{本当に is at the top level}

vs

[<真面目に><正直に>話して]いいかな？
_{真面目に is in the inner level}

or

[<(真面目で正直)に>話して]いいかな？
_{真面目で正直 has combined into a single modifier, which is at the inner level}

Answer 2

「本当{ほんとう}に正直{しょうじき}に話{はな}していいかな？」

is 100% grammatical and natural-sounding. Both 「本当に」 and 「正直に」 function adverbially.

Having never studied Japanese as a foreign language, however, I do not know if 「本当に」 and 「正直に」 are actually called "adverbs" outside of Japan. In Japan, those are called 形容動詞{けいようどうし} ("na-adjectives") in their 連用形{れんようけい} ("continuative form").

Be that as it may, one can use "those" multiple times to modify a verb or adjective.

This might surprise you, but here is a fairly long list of Japanese adverbs from Wiki. Upon hearing the word 「副詞{ふくし}」 ("adverbs"), Japanese-speakers think of those words in the list. We do not think of 「本当に」 or 「正直に」, to be completely honest. Those surely "function adverbially", but that does not make them adverbs by the Japanese standard.

It is actually me who wonders to himself 「本当に正直に話していいかな？」 everytime I see a question about "adverbs".