It has 2 FireWire PORTS (they're not outputs) because...

1. It's a standard card that works in all RM and AI series mixers.
2. FireWire is a computer data bus designed to be daisy-chained.
3. Technically the RM series doesn't have any FireWire ports... The option card has the ports and they're common to all the... see answer 1.

"Is there a reason why we can't cascading 2 RM with firewire?"
Yes, Presonus haven't written it into the firmware. There's no technical reason other than forcing AVB on us.
You can cascade 2 RMs simply by connecting them together with a short Cat5e patch lead and they'll talk AVB to each other, nothing else to buy.
But if you want to hard network to the rest of your system you'll also need to buy a Motu AVB switch...