It doesn't work much any more.

What happens is that if a particular bureau sees two pulls come in on the same day which look "too similar", it may merge them into one, because it thinks they might be duplicates of each other.

But that can only happen if you are applying for two "very similar" cards from the same bank on the same day.

It was most commonly used back in the days when Citi would let you apply for two personal cards of the same type on the same day (but would not let you apply for them a day apart or a week apart or a month apart) and give both bonuses. But earlier this year that trick ended, and Citi no longer lets you apply for two cards of any type on the same day.

Two Citi personal AA cards looked "similar enough" to the bureaus that they would typically merge them.

But there are few other examples of two "similar enough" cards that you can apply for from any other bank on the same day and get both bonuses. Most banks have "different enough" cards that the bureaus don't get confused. (And without the bureau "getting confused" about whether you two pulls are "duplicates", there's no pull merging and thus no advantage to the 2-browser trick, now that no bank allows you to apply for 2 of the very same card on the same day.)