"Holographic" presentation

Please tell me how two mono amps are said to give a more
holographic presentation than a single stereo amp. I have
had both in my system and cannot say that I have noticed
this characteristic.
As with anything else, that will depend upon the equipment. "Holographic" presentation and other aspects of quality sound and imaging are not necessarily attributable to only mono amps. You can have monos that do it well compared to stereo amps and vice versa. Put another way, moving to monos does not guarantee this or anything else (IMHO); it all depends upon the attributes of the mono or stereo amps in question, system they are put into, how you cable them, etc....these atomic statements are usually only possibly true within very specific scenarios, i.e. ManufacturerA has a stereo amp you like, they also have a mono block version of that amp. It is entirely possible there monos deliver even better sound-staging, imaging, etc....due to separating channels into different chassis, onto separate power supplies, etc....At the same time, you can find a stereo amp from ManufacturerB (or even a higher-end version from ManufacturerA) that does an equally great job at all these things assuming price is not an issue and we are only looking raw capabilities of the equipment in question.
Depends on a lot of things, how carefully the whole thing is put together, wire directionality, polarity, tweaks like resonators and tube traps, echo tunes, it's a never ending challenge to extract the largest three dimensional sphere of the recording venue possible.