I decided to focus on rewards. Got my Delta Skymiles and liked it so much, I applied for Starwood. I didn't know that Amex has a 90 day rule. (Do a search here and there are several posts that reference it) If you are otherwise "good to go" they will give you the option of reallocating your credit line. I believe their rationale is that they just deemed you worthy of an XX credit limit and now you are asking for more. My advice-- wait the 90 days.
My only experience with a SECOND card with AMEX is...clicked on "ACCEPT PRE-APPROVED CARD" (on the AMEX web site where it shows your statement)...I was DENIED for the AMEX CASH BACK CARD!!!
Even though the INCOME requirement is $0.00 when you have another card...they still DENIED ME!!! I HAVE NO "BADDIES"
I had all the AMEX DELTA one all filled out...BUT I CHICKENED OUT!!!! I COULDN'T HANDLE ANOTHER DENIAL!!!
I really love the Delta skymiles card. With the platinum you actually get 5000 BASE Miles as part of your 15,000. And you are rewarded with 10000 more BASE miles and a companion ticket in any year that you spend 25K. I don't know any other credit card that gives you base miles. They are also having a special promotion this summer from July 15-Aug 15 DOUBLE MILES on EVERY purchase. (You have to register on the Amex website for that)