OK, we need to understand a bit of quantum theory and the standard model.
The universe is best understood by reducing everything to the smallest possible number of entities (the goal of physics.) The “standard model” is our current best attempt and is based on quantum theory (more specifically, quantum electrodynamics or Q.E.D or it’s rather more bohemian cousin quantum chromodynamics, Q.C.D.)
The standard model is much like a family tree the way I teach it ... and the first split down from “everything” is into two groups called ‘fermions’ and ‘bosons’. Fermions include all matter. That means solids, liquids, gases, ceramics, polymers, you, me etc. Every single particle we know of including electrons (and there are hundreds) is a fermion. They all have one fundamental thing in common - they obey something called the “Pauli exclusion principle.”
The other group, the bosons, concerns us less in the context of your question, but includes all the known forces (called strong, weak, electromagnetic and gravity, though this last one is troublesome.)
All the known particles can be described as having a set of “quantum numbers” that determine all the properties that exist in quantum theory. Effectively this states that energy, momentum, spin and possibly even space and time exist as quantised states, meaning they cannot take any value, but only discrete ones (i.e. they are like an uneven ladder where you have to be on one rung or another, you cannot exist in between, so we would say your height is then “quantised”). The opposite would be something like an escalator, where your height could vary continuously.
The Pauli exclusion principle simply states that any fermion (particle to us, including the electrons in your question) cannot exist in the same state (complete set of quantum numbers) as another fermion. In other words (rather loosely stated, but it helps) they can be in the same place, but not at the same time, or they can be in the same time, but not at the same energy etc. It is forbidden for two or more to occupy the same complete set.
This explains why two electrons can be in the same orbit (same energy, possibly even the same position at the same time) but then cannot have the same spin. One must be spin “up” and the other spin “down” to prevent violation of this fundamental principle.
Finally, no you don’t get anything like a black hole, it is not even dangerous, just forbidden by the laws of physics.