Learn English – Comma after “that is”

Both are good, although I think the second examples are slightly better. What you are saying is :

Then (by doing this thing) this happens

Now (after selecting this option) this happens

By doing this (now) we can do this

I'm American and I can tell you that it's not dialectal. What you are really saying is "such that there were two chairs" with the parenthetical "for every person." So there are two correct ways to write it:

1. ...such that for every person there were two chairs.
2. ...such that, for every person, there were two chairs.

The former is more likely if it's a short sentence and the commas don't make the sentence difficult to decipher or ambiguous. (It may also be more common in America). The latter is more likely if the sentence is long or ambiguous without the commas. In speech you may well hear *"such that for every person, there were two chairs" but it is not correct to write it that way. You can choose to put a comma on both sides of a parenthetical or neither but you can't have one comma.