Finally, I have to say I'm unhappy with the way this thread is presented. The time limits topic has the neutral title "Time limits", so the reader feels invited to come in and debate. This one begins in bold letters "Don't do this." Sure, the small print says "we want to present guidelines, not rules", but that's the small print, after the reader has already been unsettled.
Okay, I can fix that part.
Though to be fair, it's possible someone might not even bother to read it if the words "don't..." aren't there. Unlike recognizable terms like "time limits", that one probably sounded more like some off-topic math discussion to the layman. Having the "don't" part might at least pique the curiosity of some potential readers to actually read the topic.
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Anyway, I feel like this is directly a consequence of not having a better replay scheme for handling multitasking. I also recognize it is probably tricky to design a good replay scheme that works well for multitasking.
I think it's also worth mentioning that since multitasking does not necessarily require a disjoint union design, the multitasking replay problem can still apply even without a disjoint union design.
Maybe a better, more generic way to look at this is whether some multitasking you are requiring in the level (whether introduced by a disjoint union design, or some other means) adds anything to the level besides execution difficulty. For example, Proxima's point about working out how to divide up the skills, could be something where the multitasking adds to the level.
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As for specific examples from the original games that were mentioned:
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Rendezvous at the Mountain: AFAIK multitasking is not required due to the generous time limit, and it is fairly easy to set up either side to become "safe" while you work on the other side first. So ultimately, the main complain I can see is that the level is too large for some's tastes. The skillset given appears generous enough that there really isn't much of a puzzle element either in having both sides. Ultimately the
multitasking disjoint union does not add but also does not also detract much from the level. Of course, it has to mentioned that there is no "rendezvous" conceptually if the level is not allowed two entrances and one exit, for what that's worth.
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Synchronized Lemming: this one is definitely about multitasking especially given the tight time limit. There also is some small amount of working out skills distribution due to the less generous skillset. It is also obviously not interesting if you are forced to break the level down into its individual 4 parts. The level is reasonably straightforward and short enough that I don't really see a huge pain point around the replay deficiencies with multitasking. Ultimately, I'd say it adds maybe a tiny bit to the level and does not detract too much. Given that it's not like the game is blasting the player constantly with these types of levels throughout the entire rating, I think this is an acceptable occasional change of pace. It would definitely be less okay if the level consists of four much larger disjoint areas with much more complex and error-prone paths to execute.
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You take the high road: this one does feel like it might encroach on annoying multitasking. With RR 99, 100% required and a 2-minute time limit, multitasking does seem required (though I suspect it might not be strictly necessary) and there does appear to be a few spots where fatal mistakes are quite possible. But having played it, I believe once you know exactly how you are going to blaze the path, execution mistakes are usually almost immediately apparent, and the moves from each half don't interweave too much temporally, so it seems you are rarely if ever forced to roll back the replay substantially to address mistakes. The skillset gives you more than you actually need, and each half is relatively simple, so I'd say the multitasking did not really add much puzzle-wise here.
The biggest complain I can see may be that by virtue of the forced multitasking, the player solving the level likely needs to initially focus on each half separately, then restart the level at least once to execute the two solutions put together. Of course, that complain can probably be applied to pretty much all multitasking levels. At least this one is relatively short.
Frankly for me, there just isn't much to the level with or without multitasking. If the level isn't that great to start with, does it matter whether it has a disjoint union design?