integral_result_1 and integral_result_2 are symbolic functions, not symbolic formula and not numeric values. They are functions of x_1, h, theta , but when numeric x are passed in, they are independent of x_1, h, and theta, but dependent on the undefined kappa. You cannot do a vertical concatenation of two functions, it appears. And if you could, then they would be the wrong data type (function) for an fsolve result. If you changed them so that they were symbolic expressions instead of symbolic functions then you would still have the undefined symbol kappa.
The function that you apply fsolve() to must return numeric values.