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In Phoenix, a Simulator sees other Simulators in the scene just like any other deforming mesh geometry. The loaded cache data into a Simulator represents the Isosurface of the Simulator that the other Simulators see. Simulators are Solid by default, and this way they are obstacles to other Simulators which intersect them, but you can also make a Simulator non-Solid. You can easily preview the interaction surface of each Simulator in the viewport if you enable its Show Mesh option from the Preview rollout - this will be the shape that other simulators will interact with. Note that this applies even if the Simulator would render in any other mode, not just Isosurface or Mesh.
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Using one Simulator as an Emitter in another Simulator
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The same way that one Simulator can one Simulator can be used as a plain obstacle to another, it can also be used to emit fluid from its Isosurface into a different Simulator. You just need to add a Source and select the first Simulator in it. This way, your first Simulator can run a Liquid simulation, and then be used as an emitter in a Fire simulation, so the liquid mesh would ignite in the second Fire/Smoke Simulator.
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A more advanced example of interaction between two simulators would be a cascade connection, where the fluid flows from one simulator into another. You can use cascade setups for simulations with irregular shapes where using just one Simulator grid would leave a lot of empty space and consume an unneeded amount of RAM and processing power. Additionally, if the simulation can be broken into pieces that should run one after another, then the cascade simulation would also be a good choice - this way you can iterate on the first Simulator until it's right, then iterate on the second Simulator for the continuation of the effect, and so on.
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