understanding であると同時に construction

Question

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.

Answer 1

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

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 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. 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.

Answer 2

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, 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.