What is your goal? The two-browser-type trick has up to three completely different purposes:

1. WIth Ciit AA (and no other bank and no other card that I know of), it's the trick to let you apply for two AA cards on the same day, instead of haivng to wait 2 years to get the second one.

2. With two "similar enough" cards from the same bank on the same day, it causes the credit bureau that receives two pulls to merge them into only one. (But "different enough" cards, even from the same bank, may not get merged. And cards from different banks never get merged. The merging is a "byproduct" of the bureaus trying to remove "duplicates" from your report, and they only remove them when they can't tell whether or not they might be duplicates. So if you apply for two "similar enough" cards that the bank sends the same pull information for both, they may get merged.)

As an example of "simialr enough": Two different Citi HHonors cards are "similar enough". Two difeferent Citi AA cards are "siomilar enough".

As an example of different enough; A Citi HHonors card may be "different enough" from a Citi AA card to show up different on a pull, and thus not get merged. (Ie, cards form the same bank but of totally different "affinities" may generate different-looking pulls that don't get merged.)

And a personal card and a business card (of otherwise the same card) are often "different enough" to generate different looking pulls and thus never get merged. (But I don't know specficially for SPG Amex.)

3. Wehn applying for two identical cards at the same time (hard to do with banks other than Citi AFAIK), using two browsers, or using two Private (aka InPrivate) sessions in the same browser, can keep the cookies from the two (otherwise identical or "too similar") apps from getting confused.

These are the only reasons to even consider a two-browser-type trick. There is no need to use two browsers if the cards are from different banks, as none of the reasons stated above can apply if the apps are from different banks.