Cost is an issue in principle since a second unit in parallel (or for boost) wouldn't be used much unless the weather required it. I suppose you could look at it as extra "insurance" cost...or having a backup generator in case the primary unit failed. I could buy a second Honda and not use it unless it was needed, but that is another $5k just sitting around in the garage. At least they have good resale value though.
The Victron Multiplus looks like it would be just as (if not more) expensive since I'd require 2 units for split phase, plus a large battery to provide the boost current.
I suppose I could also consider paralleling a smaller Honda like this guy did but I don't think Honda advises it and it could potentially be bad for the inverter module: