When was the last time you flew SP IFR John? There are some great things about multi-crew, BUT my impression is that the sum of all cockpit workload in a two-crew cockpit is much higher than the workload of one pilot in a single pilot cockpit, in other words, doubling the number pilots does not halve the workload of each pilot.
The reason being, as far as I can tell, is that as a single pilot there is only one person who needs to have a mental picture of what is happening, and that person is you, the pilot. When the situation changes, your mental picture changes and you change your plans accordingly and you don't to tell anyone about it except maybe ATC or other traffic. In a multi-crew cockpit, you need to communicate with the other pilot to ensure that you both have the same mental picture. This means communicating your thoughts about what is happening and what you plan to do, and sometimes you find out that the other pilot's mental picture doesn't match yours and you need to have a discussion to resolve the differences ("ah, that aircraft is coming in from the south, I thought he was coming in from the west.") This all takes time and can make a situation that would be simple for a single pilot, become more complex for the multi-crew.
So single pilot can have a high workload but it doesn't require any in-cockpit communication. On the other hand all that communication can highlight errors in your own mental picture so hopefully you end up with a safer operation when you have two guys working together rather than just one.
Note I'm not saying that two pilot crews need to talk all the time, a lot of the communication is covered by SOPs in terms of what is expected to happen and when.
My point is that suggesting that there are times when one of the pilots in a two pilot crew may have a high workload in no way implies that that same pilot would have trouble in a single pilot situation. They're different environments that can't be directly compared.
On to the workload of the PM/PNF when the PF is hand-flying. It's not so much that the PNF will have too much work to do, it's just that the PNF will have a much higher workload than the PF (after all, hand flying is pretty easy for those that maintain the skill.) So the PF needs to be mindful of the fact that if they're going to hand fly a departure or approach, they don't do it at a time when it would be most wise to share the workload more evenly.
A good pilot will use a level of automation appropriate to the situation. One of the things that should be considered when deciding what automation is appropriate is the balance of workload across the crew. I think that's all Parabellum was saying.
Incidentally I think the hardest part of hand-flying a multi crew aeroplane is learning to accurately tell the PNF what you want them to do.