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20M spacing seems massive. I am not sure how you got this estimate.
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It was a guess because I don't know the bandwidth of the first IF filter in the Siglent analyser. I think it will be less than 20MHz but probably not by much. If the tones are spaced at a wider spacing than the bandwidth of the first IF filter in the Siglent analyser, then the second mixer will not see both tones at the same time. Therefore, the first mixer and the IF amplifier after it will dictate the input TOI.
This might improve the input TOI, but a lot depends on how well the IF1 amp performs when feeding the test tones into the stopbands of the IF1 filter that follows the IF1 amplifier. All this stuff is hard to judge, but the gain distribution in the Siglent analyser does look a bit unconventional to me.
The HP 8568 analyser overcomes this issue by using an attenuator at the IF port of the mixer (to improve the IF port termination here) and the first stage of amplification is after the second mixer. The HP 8566 uses a directional filter (a bit like a diplexer) in place of the 6dB attenuator and this costs more and should perform better.
This gives a consistent input TOI right through the first two mixer stages. That's how spectrum analysers were designed 40-50 years ago. Modern analysers do now tend to have an amplifier after the first mixer, but it typically has a low gain and a very high input TOI. This IF1 amplifier stage improves the DANL by a few dB for one thing. It can also introduce some much-needed reverse isolation between the first two mixers. The IF1 amplifier in the Siglent looks to have >15dB gain at the IF1 frequency and this seems very high to me.