I know that kit well but would need to know what disks you have and how you intend to use them to give a full answer.
For instance if you have 8 disks and want to use them all in one R5/R6/R10 array then I'd strongly urge you to use just the P800 as that way the array can be entirely managed in hardware. You'd have to use software RAID to link them them across two controllers.
If on the other hand you wanted two separate 4 disk arrays then by all means use both controllers, if you select the right PCIe slot for the P800 you shouldn't be bus-sharing and would see the benefit of two independent controllers.
Let us know these extra details and we can perhaps help more ok.