Here are a few snapshots from the song "Alborada del Gracioso" by Maurice Ravel. How do you play two notes at the same time while doing a glissando (and only using one hand?) To my understanding, one must slide two fingers at the same time to achieve this, but how would you place your fingers to achieve this? Thank you!
Hard to describe exactly without a picture/photo, but you place the two fingers you find most convenient at the usual angle for a glissando, keeping them parallel and a fixed distance apart. Then you try it, and it sounds awful, but you practice. And practice. And more practice, until it begins to sound like a real glissando. Then you need more practice.
There is never a simple "correct" answer on something like this, but I seem to find it easiest to use 1+3 on the way up, and 1+2 on the way down. (I used to play this once, not very well.)
I think a light touch is essential for this: it really doesn't matter if a note here or there doesn't sound, whereas if you try to make it sound like playing scales you will end up with bruised fingers. You could start by just running your fingertips silently over the keys, to get a feel for how fast they will be moving to fit into the tempo. And the most important notes are the top and bottom (start and finish) ones, so the next step might be to play just those, with a sweep over the keys in between.