Are Wizardry 8 spells from two party members cumulative?

Solution 1:

The spell Magic Screen doesn't stack. Casting it again just replaces the old buff with the new one.

For example I loaded up an old save and had my Valkyrie cast Magic Screen at level 3 and a level 3 buff was placed on the party as you can see in this screenshot:

Then I had my Bishop cast Magic Screen at level 4. The previous level 3 buff was changed to a level 4 buff:

As you can see by the tooltip in the upper right corner the Magic Screen buff affecting the party is at level 4.

I would assume all buffs and debuffs work this way. Wizardry 8 is an old school game, even more than it being 16 years old would suggest. It's using mechanics that haven't changed much since the original Wizardry was released in 1981. The concept of "stacking" spells or other effects is relatively modern invention.