Multiple PSD in same graph - Matlab

I intend to plot multiple Power Spectral Densities on the same graph. I am using the following to plot the power spectral density for a single signal.

    hss = dsp.SpectrumAnalyzer('SampleRate', Fs); 
    step(hss,rx);
    release(hss);

However, if I were to plot another signal in the same spectrum analyzer using hold on doesn't seem to help

    hss = dsp.SpectrumAnalyzer('SampleRate', Fs); 
    step(hss,rx); hold on;
    step(hss,tx);
    release(hss);

Could someone guide me on how to go about with this.