understanding であると同時に construction


> やけに高く響くその靴音は、スバルにとって焦燥感の原因 **であると同時に** 、心の拠り所でもあった。

So I basically translate it as **"For Subaru, those footsteps that echoed
absurdly loud were, at the same time, the source of his impatience and also
the one thing he could rely on."**

What confuses me here is **「であると同時に」**. It is the same as **「だと同時に」** , right?
Either way, I don't understand this construction/grammar.

I was thinking that maybe it would be ok to rephrase it like this:

> やけに高く響くその靴音は、スバルにとって **同時に** 焦燥感の原因 **と** 、心の拠り所でもあった。

Also, despite the grammar points, is my translation close enough? Those are
the first lines of a book I'm reading btw.




デジタル大辞泉 says this type of と同時に is a special case where the whole combination
of と同時に works like a conjunctive particle:

> ### [同時に](https://dictionary.goo.ne.jp/jn/155915/meaning/m0u/)
>
> ３ （「…と同時に」の形で **接続助詞的に** 用いて）  
>  ㋐…すると同じ時に。…するとすぐに。「夜が明けると同時に出発した」  
>  ㋑…とともに。…だけでなく、加えて。「他人に厳しいと同時に自分にも厳しい」

I may be wrong, but I feel this is a remnant of classic grammar, and there is
a [hidden nominalization](https://japanese.stackexchange.com/q/6714/5010) in
this construction. ～であると同時に is interchangeable with ～である **の** と同時に, where の
is a nominalizer and と is [a comparison target
marker](https://japanese.stackexchange.com/a/57077/5010). So Aである(の)と同時にB
basically means "while (being) A, (also) B", "in addition to being A, (also)
B", "A, but at the same time, B" or "not only A but also B".

However, ～だと同時に is ungrammatical. If I understand this correctly, this is
because the verb/adjective before と must be in the attributive form (連体形). だ
is the predicative form (終止形) of the copula.

同時にAでありBで(も)ある is semantically similar, but it is simply "both A and B at the
same time", where the two items are treated equally. Aであると同時にB sounds a little
more emphatic or rhetorical to me.




The basic idea behind the grammar Aと同時にB is that A happens at the same time as
B or that something or someone is in both state A and state B.

According to a [Dictionary of Intermediate Japanese
Grammar](https://core6000.neocities.org/dojg/entries/360.html), the pattern is
as such:

> Noun + と同時に
>
> Verb plain + と同時に
>
> Na-adjective/Noun + であると同時に + Na-adjective/Noun + でもある

Here it's saying that for Subaru, the 靴音 was both 焦燥感の原因 as well as 心の拠り所.

EDIT: Okay, after some more prodding from Chocolate and some
thinking/consulting with others, I think I figured out the difference between
noun+であると同時に and noun+と同時に.

noun+であると同時に means that the noun is describing the state of the subject/topic.
So in this case, 焦燥感の原因 is describing something about of その靴音.

noun + と同時に doesn't have this relationship to the subject/topic.

