I was confused for a minute on how it's both _geostationary_ and _over Europe_ -- you can't be geostationary if your orbit is not over the equator!
Turns out[1] the MTG-S1 satellite is in fact geostationary and parked at exactly 0°00'00"N 0°00'00"E (off the coast of Ghana), 42164 km up from the center of Earth, it's just pointing at Europe at an angle.
But you are right, [1] confirms "0° longitude".
[1] https://user.eumetsat.int/resources/user-guides/mtg-in-opera...
Presumably it's an intentional choice to put it at such a round number, rather than any scientific benefit over it being, say, 10km west or east.
[0] A few sites give 10km as a standard minimum separation for geostationary satellites. That theoretically allows a thousand of them in the 100km cube, but I am guessing a lattice of them every 10 km in all 3 dimensions would not be manageable.
But some geostationary satellites are close enough so that there can be failover without adjusting receiving antennas on the ground.
So you can of course keep them dense around the equator. Probably very close down to hundreds of meters (if not less) if you coordinate the station keeping. After all the forces that push or pull the satellites out of orbit (tidal forces and particle streams) should be very similar for close neighbours. Problem is that you have to share the bandwidth of the up- and downlink then because the dishes of the groundstations cannot focus so sharply.
Given that, and redundancy put aside, one bigger satellite with more payloads would usually be cheaper than two smaller ones without any disadvantages.