I found a picture of a heat pump with dual compressors.
You can clearly see that the compressors are simply plumbed in parallel, with no valves. This is not a cascade of two heat pumps, this is one HP with two compressors, everything else is common.
From what I've read, the controller turns on only one compressor at a time (in an alternating fashion). Some controllers keep an account of how much on-time each of the compressors has had and turns on the one which was used less often. When one compressor takes too long to do its job, then the second one kicks in.
So building one of these can't be much harder than a normal HP: two identical compressors plus everything else sized for a compressor twice as big. Making the controller would be a little harder, but still doable.