It was particular to the circumstance. Having a shared witness doesn't always mean "sharing" the node or the account which gets paid. You have to trust a particular person to hold the key and distribute the payment. What that means is there can be a dynamic shift in power relations within a team or partnership where somebody could simply stop paying you whenever they feel like it.

That dynamic shift can mean every promise before the agreement was made thrown out the window, especially if all you were really needed for was to pull in the initial votes from the announcement of your partnership. It works for some people. I would say beware of who you trust, and trust your gut first.

do you know if one "single witness" could, in essence, "split up" his or her mining obligation "across" multiple--otherwise separate--CPUs? From what I know about mining

I think it's better to remain one witness and if you share the rewards of that witness be open about their lower positions as promoters and what the reasons are they are supported by your witness rewards. Currently the blockchain doesn't distribute witness rewards to multiple accounts. So it has to be powered down to share the block rewards.