There are many situations when multiple comps are used with no problems. The problem with Hallowell is that they used a Bristol comp and made changes to it that Bristol did not condone. It was a troublesome system from day one and they left a number of people in the lurch with warranty issues.
That said, there are many ways to get great low temp performance without two comps. Large evaporators, liquid or vapour injection and VSD are just some of them. The biggest single improvement to an ASHP is to use the lowest output temp possible which means floor heating and not forced air. The system (house insulation and floor tubing) needs to be designed for 30C max temps and with this the COP will be acceptable.
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