hi there!
there is only one way to learn to read: read a lot! to myself, read one clef or read an orchestral open-score is preety much the same. of course that speed, amount of simultaneous notes, patterns, (everything indeed) counts and makes a huge difference, but i never realized any difficulty particularly associated to simultaneous reading.
btw, it´s much more difficult imo to read a single staff by boulez, than to read an orchestral minuet by mozart.
i think you just get used to read it. start with simple material, and go on progressively. i use with my students (and did use it to myself as well) the mikrokosmos by bartok (because it´s never obvious, and uses anything from monophony to canon).
about several staves, start reading simple voice+piano material, playing both parts (there are several works where the composer already write in the piano a voice doubling, so those one don´t count

). then, procede to instrumental trios, quartets (haydn´s slow movements are the most), and so on.
however, just notice how many different and interesting approaches there are above. i think any pianist must develop her/his own "technique" to read, because it´s so related to the piano skill, to musical knowledge (the more i understand harmony, cp, etc, the easy to read), to focus, etc, that is impossible (at least i think) to reach universal rules or a perfect way of doing that. try everything and try it very much!
hope it helps a bit!