The second x16 slot on the P35 board is at x4 speed only. And remember this is PCI express 1.0 not 2.0 like the p45 board. So the p45 x8 slots process 4 times the bandwidth of the p35 x4 slot.
The specs on the p35 are a bit strange in that they do state "Ultimate graphics performance with dual PCI-E x16 interface" but no where do they state Crossfire ready or SLI certified as normally would be done if intended for those uses. I guess you are supposed to understand what the abscence of such designations imply - and they do state the limited bandwidth of the second x16 slot.
But you will get more bandwidth out of one x16 slot fully utilized than you will out of two x4 slots - approximately twice as much. So I think you want to forget using two cards and get one big one. And after all, wouldn't you like to have a big one?