if you are using your Mac on Leopard (10.5.x) the H8000FW firewire driver doesn't support yet this OS X version. It's good for 10.4.
Firewire aggregation should be possible in Logic if you are on 10.4.
Please also consider that the MacPro firewire400 and 800 ports ALL share the same bus (NOT so Pro!). So additional FW cards are the real pro solution to limited bandwidth Apple poor design offers.
all the best