To get memory mapping across systems you'd want to look into infiniband (https://en.wikipedia.org/wiki/InfiniBand) but the adapters generally aren't cheap. Common to find in super computer clusters.
If you have a desktop CPU with 2 memory channels and 2 DIMMs per channel, then the 2 DIMMs on each single channel share all of the address and data lines, there's just a chip select difference to pick which DIMM you're addressing. There is some extra loading on the lines, since you have 2 DIMMs, hence the supported speeds are usually slightly lower, but you only add 1-2 more actual PCB traces to have 2 DIMMs per channel vs 1 DIMM per channel. In contrast, adding another memory controller would add upwards of almost 100 additional PCB traces from the CPU.
If you have a desktop CPU with 2 memory channels and 2 DIMMs per channel, then the 2 DIMMs on each single channel share all of the address and data lines, there's just a chip select difference to pick which DIMM you're addressing. There is some extra loading on the lines, since you have 2 DIMMs, hence the supported speeds are usually slightly lower, but you only add 1-2 more actual PCB traces to have 2 DIMMs per channel vs 1 DIMM per channel. In contrast, adding another memory controller would add upwards of almost 100 additional PCB traces from the CPU.