do you know how to translate.

A,B正副議長以下...参集し御諮詢相成りたるー,元号制定を上議し慎重審議の結果.

以下= does this mean "and so on" here?
相成り means "become"
And does anybody know the meaning of たるー in this context?

Thank you!

たる is the derived form of たり.
たり is in Classic Japanese an auxiliary which means a perfect tense or an affirmation of fact.
Grammatically, it succeeds the continuative form of verbs and followed by a noun or in general a nominal.
In the sentence you show, it means “has become” or simply “became”, literally, ie. used as present perfect.
You will surely understand the meaning of the whole sentence and will be able to translate it. The word used seem to be a little classical and difficult, but the sentence itself is not a difficult one.

I don't think this sentence is current Japanese.
以下 means "and other persons" and "other persons" have to have lower positions than 正副議長.
諮詢＝諮問、discuss or tell opinions
相＝お互い
成る＝する
たる＝した(past tense)（ところの）
上議＝議題にする（上げる）

相成る（あいなる）is a formal expression of 成る（なる）.
It is not the combination of 相 and 成る, so it doesn’t mean 互いにする.
御ご of 御諮詢ごしじゅん was used to express this 諮詢(inquiry) was an act of the emperor. Generally, the acts of the emperor were expressed in this way, as if they were made (or became realized) in a natural and impersonal way, because the emperor was conceived as someone who knew with authority 知らす, not someone who governed with power 統治する.
I don’t think it is necessary to explain to a person who can treat this kind of old formal Japanese, the difference of Japanese and English usage.

相成る（あいなる）is a formal expression of 成る（なる）.
It is not the combination of 相 and 成る, so it doesn’t mean 互いにする.
御ご of 御諮詢ごしじゅん was used to express this 諮詢(inquiry) was an act of the emperor. Generally, the acts of the emperor were expressed in this way, as if they were made (or became realized) in a natural and impersonal way, because the emperor was conceived as someone who knew with authority 知らす, not someone who governed with power 統治する.
I don’t think it is necessary to explain to a person who can treat this kind of old formal Japanese, the difference of Japanese and English usage.