You can put as many spirit cubes on your weapon up to the customization limit (so be careful about loading up on simple cubes).
all of the spirits can be put on a weapon and all effects will be achieved. You can have a fire ice weapon that deals extra damage and slows.
You can only put spirits of the power level or slightly lower.
With 1 unholy spirit cube on my crossbow, I got additional .1HP back per hit every 20th combo (so 200 combo is 1hp per hit). With 2, this effect doubled. I havent been able to test more because usable spirit cubes are a bit rare.