Is the spa going to be connected to the pool? Sharing equipment? Will the spa spill into the pool?
If they are attached, usually a skimmer is not need in the spa as debris will spill over into the pool when filtering the water.
There would be no harm in adding a skimmer in the spa if plumbed properly, even if it is attached to the pool.
If it is completely independent of the pool, then you certain will want a skimmer.