One way to contruct a subtle and complex ambience in a mix is to combine two different approaches to reverb. Going about this in an informed, deliberate way will result in a much more refined and appropriate sound than by simply stacking two different reverb algorithms (either in parallel or – heaven forbid – serial).
One way to approach it is to think about foreground and background. Often using a single reverb results in an ambience that sits primarily in the forground (resulting in a shallower mix) or in the background (resulting in a relatively dry foreground). Using two reverbs might allow a mix the benefit of both the foreground ambience (for softness and blurriness) and background ambience (for depth and spaciousness). One way to do this is to use a plate for the foreground ambience and a hall for the background ambience. This will be most coherent if foreground sounds are mainly (if not exclusively) sent to the plate, and background sounds are mainly (if not exclusively) sent to the hall. This approach is useful if the mix calls for a lush ambience with a three-dimensional quality to it.
Another approach is to combine short and long reverbs. This can be appropriate if the song calls for a long deep ambience, but there’s no middle ground between too dry and too lush for some sounds. This way, some textural background sounds and feature sounds would use the long reverb and other sounds (particularly more percussive/articulative sounds) would use the short reverb. A hall or plate would be suitable for the long reverb, and a room or shorter plate might be suited to the short reverb. For a more unnatural sound, use a thick modulated hall for the long reverb and a non-linear reverb for the short reverb. This approach is useful for complex mixes that don’t need to have a particularly realistic acoustic sound, such as electornic music and ‘studio’ music.