FFT of two sets of samples vs FFT of sum of samples
Fourier Transform is linear, hence if $\mathcal F[x(t)]=X(f)$ and $a$, $b$ are complex numbers, then:

$$\mathcal{F}[ax(t)+by(t)] = a X(f) + bY(f) $$

So in your case, simply sum the results together.