I'm not familiar with the term LUT.
The Quadra 4000 and the Quadra FX 1800 both have two displayport slots.
You do bring up another option.. I could put the Quadra FX 1800 in another PCI Slot and run each monitor independently. There is an open space.
LUT (look up table) is where the information is stored that profiles each monitor. You have internal and external LUT's. Monitors like yours require and external LUT provided by the video card. If you don't have two, you will never be able to have two profiled monitors working side by side. You could rig it to work one or the other, but not both at the same time.
Did you run the utility on each monitor? This would be your first step.. verify if you have a LUT available on each monitor or not.
How many physical ports you have is unrelated to how many LUT's your card has.
And yes, you could put two video cards side by side.. and since you already have both it would be a good option to try, but it's always a better idea to run two of the same cards to avoid issues with software.