I'm not sure what you mean by needing to save dummy dives in Subsurface. I have not done what @KenGordon described, where I download my actual first dive, then plan my second. But, I think it would work, based on what I have done.
What I have done is planned a day with 2 deco dives. I plan the first dive first, including entering the actual date and time I anticipate doing the dive (obviously an estimate). When I have it planned out how I want, I save it which enters it into my dive log.Then I plan another dive. There is nothing connecting it to the first dive other than that I enter the same date and a later time. Subsurface reflects residual tissue loading from the first dive in the second dive automatically based on, as you said, the dates and times of the dives.
So, to @KenGordon's point, once you plan a dive and save it, it appears to me that Subsurface treats it the same as any other dive in the log. So, when planning the second dive, as far as I know, Subsurface would reflect tissue loading from the prior dive, whether the prior dive was actually downloaded from a dive computer or whether it was one that was entered into the log via the Planner.
To summarize: I have planned 2 deco dives for the same day, specifying the surface interval by specifying the day and times of the dives, and verified that the second dive's plan does reflect residual tissue loading from the first dive. I have NOT downloaded a dive immediately after doing it, then attempted to plan a 2nd dive where the plan accommodates residual tissue loading from the downloaded dive. But, based on the way the Planner works, I think it would work the same for planning the 2nd dive, no matter which way the first dive got entered into the log (i.e. via download of actual versus via the Planner).
I will have to try this at my next opportunity. I think the process to satisfy me that it works like I want would be: Take my laptop to the dive site. Do a dive. Go into Subsurface and plan a dive for 1 hour in the future. With the 1st dive not yet downloaded, Subsurface should produce a result based on an SI of >24 hours. Note the results (i.e. calculated NDL or deco obligation). Then download the 1st dive into Subsurface and re-plan the 2nd dive, still for the same start time. If Subsurface works as we would like, then the 2nd dive would have a reduced NDL or increased deco obligation after the 1st dive is downloaded.
In real life, I might plan 2 dives for a day, then want to re-plan the 2nd dive after I do the 1st dive, so that the new plan is based on what I actually did, instead of the square profile I planned for. In that case, I would probably want to delete the 1st dive, which was produced by the Planner, then download the actual 1st dive. Then re-plan dive #2. This is, of course, assuming I saved my log file after planning the 2 dives in the first place (which I typically do, for this kind of thing). Otherwise, I would just do dive 1, download, and plan dive 2 from scratch.
