Most true multi-zone systems have a lower turndown ratio than single zone systems. if you choose a dual head setup, make sure that doesn't affect you negatively.
The problem I see is the refrigerant and oil trapped in the closed off section. for it to work well, I believe you'd need to "pump down" the circuit you aren't using. otherwise you have will have little control over how much refrigerant is where, and will likely lose capacity.
if you proceed, you would want to make sure you don't trap oil in your manifold, and be sure to choose valves and piping rated for refrigerant use.
have you considered two single systems? that would get you some redundancy as well, while maintaining simplicity, not to mention warranty support wouldn't be compromised (assuming this matters to you - on a really hot/cold day, it might).
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