Make sure to only engage in the erotic de-mag wand dance upon confirmation that there are no innocent audio/video tapes or mag-strip ID/credit cards hanging around in close proximity, as those are the items most susceptible to the corrupting influence of strong magnetic fields.
I'm no expert on the mysteries of magnetics, but doubt the heads of one deck will effect another that happens to be stacked above or below it, simply because if that was the case a tape set on top of or immediately adjacent to the deck would be even more strongly effected, and AFAIK that isn't an issue.
But it's easy enough to clean and de-mag whichever deck is about to be used simply as best practice, well away from any innocent tapes.
I still have my now 40 year old Radio Shack demag wand buried deep somewhere. Last I used it several decades back I was specifically leveraging its magnetic corrupting abilities. I screwed up some old cathode-ray-tube color TV's pretty good with that thing and a handful of of old hard-drive magnets doing a projected light show using an old overhead projector lens afixed to the TVs with a cardboard box. Whoa! double rainbow!