It is not clear to me if we can observe matter infalling to a black hole. When it is stated that time stretch to infinite for an external observer, then we should see a kind of accretion or jam at the very border. Like two compenetrating objects at the same place. I hope this is naive.
To be precise: think of the crash test mannequin that divulgative science often send to a black hole. Ignore spaghettification. Do we really see that mannequin "frozen" and never passes throughout the horizon?
If so, what happen to a second mannequin if it happens to it to fall at the same spot?
Or there is no the same spot as for the infalling matter has already shifted the horizon?
May be the mannequin is not the best testing particle. Feel free to replace mannequin with atoms or particles and so forth. I have been pictorial for sake of clarity.
Edit: the opening sentence refers to direct observation. Not to the observing of radiation due to the acceleration towards the black hole like that emitted by a hot accretion disk.