Just my limited experience, but I mocked up my planned height and came out with measurements to base construction of my two level layout.
In the end, the upper level was a good height for viewing while standing, and I bought a mechanic's stool for viewing the lower level. The separation ended up being 20" and the max width of my shelves will be 20" on the upper level and 24" on the lower.
The stool has four caster wheels and the seat adjusts in height very similar to an office chair. You can roll along with the train when it's on the lower level and walk along when it's up top.
I highly recommend a mock up to test comfortable heights and shelf widths. Remember that there will be scenery above each base level, so allow for that, both in width that can be easily reached, and to allow for any vertical scenery (both up and down) on both levels.
Due to limited space I plan to use an elevator to change levels, short trains will accomodate this.