The same single function 'x' cannot in general satisfy two different differential equations, so you need to solve these equations separately.
It should be mentioned that you do not really need to use matlab's numerical ode functions on the particular equations you ask about. Their solution is well known in mathematics. If k/m is positive then x is of the form:
x = A*cos(sqrt(k/m)*t)+B*sin(sqrt(k/m)*t)
If k/m is negative it is
x = A*cosh(sqrt(-k/m)*t)+B*sinh(sqrt(-k/m)*t)
The constants A and B are determined by the two initial conditions which x must satisfy.
