Hi,
You can certainly place two cards of the same type into different slots.
The only system which has any sort of 'good practice' with regard to where you place PCI cards is the E450 (which is quite likely what you have). You can place the cards anywhere but there are different speed PCI slots in there, as well as load balancing. All 10 PCI slots are handled by 3 chips (PSYCHO chips if you're interested). So if you're only putting 2 or 3 cards into the system, it's good practice to balance them across the different PCI controllers (PSYCHO's).
If this is the case, email me and I'll send you the slot/controller information.
Hope this help,
Darren Rathbone