I think you got your wires crossed. The PenFed Amex is not a "real" Amex card. It carries the Amex branding, but it's backed and issued by PenFed whereas the cards you find on Amex's website are backed and issued by Amex. Moreover, the PenFed rewards program is completely different and entirely separate from the Amex Membership Rewards (MR) program. You can't trade or combine points between those two programs.
However, you CAN have more than one card that participates in the MR program. I currently have the Platinum card and the Premier Rewards Gold (PRG) card. I think Amex would be perfectly happy giving me a couple more card should I want them.
There are currently about 20 different cards that are offered in both business and personal forms that are apart of the MR program at 3 different levels. Only 2 of the levels allow transfers to airline and hotel partners. To learn more about the different cards offered and about the different levels of MR check out Amex's website here