It'll be a combination. Bottoming out can be damping or spring
Getting the right spring rate in this instance I would say be more of a compromise than usual unless majority of use is definitely two up.
Two up for me is not really fun so I wouldn't care too much for that and so set up would be biased towards solo. You could fraff around with shocks off other bikes (and be sort or maybe not) or get a shock specific to your needs. . An emulsion shock built for you would set you back a little bit over £300 or some places do have deals on Ohlins shock cheaper than list price if you shop carefully.
At this juncture I'll say you have to think how about how much you prepared to pay for a solution and go from there.
I know what I'll do myself
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Cheers
Ben