I was using the 2023 RS Berlin guide on a recent trip to Potsdam and it was quite incorrect, so posting late July 2023 experience.
The most important correction is that the Neues Palais requires timed tickets (no mention of this in RS) and the times CAN ONLY be reserved in person at the Neues Palais ticket office. A person can buy a ticket for Sans Souci online with a time slot, and intending to add access to the Neues Palais to it buy a combination ticket. All the time slots for the Neues Paiais get taken before the end of the day. Thus it is entirely possible to buy an advance ticket for the Neues Palais and BE UNABLE TO USE IT because all the times are taken.
The Sans Souci ticket office cannot reserve Neues Palace times. The palaces are 30 minutes apart either by bus or walking.
Only 120 per hour are allowed to tour the Neues Palais in groups of 40 via live guide. Sans Souci has continuous audio guide touring.
Suggest reserving Sans Souci after lunch and starting with your combination ticket first thing in the morning at the Neues Palais. That’s the only way to ensure being able to tour both palaces. Repeating that buying a ticket to both palaces in advance does not ensure entry.
Other RS issues.
The buses for the palaces leave from the south end of the very large Potsdam hauptbahnhof opposite the town (north) side, signage is poor.
Palace buses leave from lane 10 (not 4). [may be temporary, ongoing construction.]
There is perhaps a nice art museum at Sans Souci not mentioned by RS and included with the combo ticket. I wish I had budgeted time for this but missed it. It has at least one Caravaggio.
A note that the Neues Palace is in only fair condition inside. Some may be disappointed. For penny pinchers the toilets cost twice as much at the Neues Palace than Sans Souci.
Adding that on today’s journey by train Dresden to Prague I had reserved seats via DB on wagon 255. The train pulled into the station short 2 carriages including 255 so we had no seats. I complained to the DB uniformed employee and she said “your problem” and walked away. I’m beginning to think Germany is the new Mexico.