You can set FrequencyResolutionMethod to 'Window length' to manually adjust the length of signal used to compute the spectrum, e.g.,
Fs = 2500;
Sineobject1 = dsp.SineWave('SamplesPerFrame',1024,'SampleRate',Fs,'Frequency',100);
Sineobject2 = dsp.SineWave('SamplesPerFrame',1024,'SampleRate',Fs,'Frequency',1000);
SA = dsp.SpectrumAnalyzer('SampleRate',Fs,'NumInputPorts',2,'PlotAsTwoSidedSpectrum',...
false,'ChannelNames',{'SinewaveInput','NotchOutput'},'ShowLegend',true,...
'FrequencyResolutionMethod','WindowLength');
step(SA,step(Sineobject1),step(Sineobject2))
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