When more than one spectrum is applied, there will most-likely be a relative displacement component to the load. (One structure or floor may be wagging left while the other is wagging right.)
This displacement set may be the cause of your high stress. If it is the largest of all stress components, it will be indicated by "P" (for pseudostatic) in the stress report.
You specify this relative displacement in the last column of the spectrum definition. If you do not specify a value, CAESAR II will calculate the value from the response spectrum. In many cases the values extracted from the response spectrum are excessive.
(Check your seismic displacements.)