大きな深刻な問題 two na-connected attributives

Question

The accepted answer under this question suggests 色々な便利な道具 is natural but it is most likely due to 色々 being an exception. I have just heard 大きな深刻な問題 on a Japanese TV program. And Google seems to show more examples with this phrase:

山梨県の長崎知事は19日、新型コロナウイルスの検査体制の拡充などを発表する記者会見の中で、東京オリンピック・パラリンピックの開催について「国民や県民の健康に極めて大きな深刻な影響を及ぼすような感染状況であれば、オリンピックをやっているどころではない」と発言しました。(source)

いま、まさに、これが大きな深刻な問題になっています。(source)

まあ、1歳児のこのようなちょっとした問題はいいとして、思春期になってくると、もっともっと大きな深刻な問題もたくさん起こすのではないでしょうか。(source)

If I'm not mistaken in 学校文法, 大きな is a 連体修飾詞, and 深刻 a 形容動詞. Is that the reason why they can be connected with な?

Answer 1

大きな is not an i-adjective nor a na-adjective but a fixed rentaishi that never conjugates. It doesn't have a te-form to begin with, so 大きな深刻な問題 is the only option if you use 大きな (大きな modifies the noun phrase 深刻な問題 as a whole). You cannot say 大きで深刻な問題 just as you can't say 色んで便利な道具.

大きい深刻な問題 is not incorrect, but 大きな is much more common than 大きい when used attributively. If you want to emphasize something has two properties at the same time, using くて or で may be more explicit on that, so something like 大きくて深刻な問題 might be better. かつ is even more explicit.

With that said, I also feel 大きな深刻な or 大きくて深刻な sounds fairly redundant... What is a serious problem that isn't big?

Answer 2

If I'm not mistaken in 学校文法, 大きな is a 連体修飾詞, and 深刻 a 形容動詞. Is that the reason why they can be connected with な?

I think this is one of the contributing factors. Two 形容動詞 in the same context would be much more commonly connected with で or かつ, as in 冷静かつ迅速な対応, instead of 冷静な迅速な対応.

But more generally, the sequence of two attributives (including verbs) does not make the sentence ungrammatical automatically. It can be understood as a coordination of two, although it might happen mostly in speech. If it is a spoken sentence, you might transcribe it as

これが、大きな、深刻な問題になっています。

It could be an attempt to correct what you have just said. (I think this is what 学校文法 would discourage, since sentences that include correction can be seen as broken in that approach.) It could also be intentionally done to have an emphasis on the second part, somewhat similar to "I did X, or rather [more accurately said], Y".