My understanding (which could totally be wrong or incomplete!) is...
The switch chooses between the active play head: short, long, or both. "Short" selects the play head that is closer to the record head for shorter delay times, "long" selects the play head that is further from the record head for longer delay times, and "both" uses both play heads (like a multitap mode). This isn't too dissimilar from some of the other Strymon delays.
The big difference is that there is the "decaying static charge", which is basically an always-present repeat with a fixed regen amount—its feedback is always on and is separate from the main regen control, which controls the feedback of the selected play head(s). So, in practice it's more like a third play head that has a fixed feedback amount (and possibly different filtering?)
The rate of all three (both playheads and the static charge repeats) are controlled by the "time" control, which simulates changing the drum's rotational speed, with a range of 72ms to 290ms for the short play head, 155 to 620ms for the long play head, and 200ms to 800ms for the static charge repeats.
So it's true that the ratio between these three is not directly controllable, but it's all tied to the time knob, and designed in such a way so as to create atmosphere and a reverb-like effect (especially with the main regen control turned up). I'd think of it sort of like a golden ratio delay mode where it's part of the effect rather than something you'd directly control.
It would be cool to have the direct control as an alt option, but that's maybe a different delay effect, like something closer to a normal multi-head tape delay. The OBNE Black Fountain Stereo then would probably be perfect, because it's basically a two head tape delay, with both heads having independent time controls, and without the static discharge thing, but with the wonky "fluid" modulation, and a diffusion control that can add in some of that ambience the static discharge is supposed to give. It also adds tap tempo and subdivisions, and a clipping control.
The more I look at both, I like them both! If comparing the Black Fountain Stereo to the Strymon stuff, I'd see it more as an El Capistan competitor that has a murkier texture but is similarly flexible, while the Olivera is a very specific effect that, for a lot of people, might be better off as a single mode on a larger pedal... but is going to be absolutely perfect as an almost-always-on for others.