Note with two stems in snare drum part [Ravel's Miroir]: What is the result?

Speaking as a physicist who's had to deal with the series expansion of "drum-head" vibrational modes, I can offer this:

When you hit one spot on a drum head, you are exciting resonant frequencies based on energy you poured into that location. When you hit two spots simultaneously, you will get some mix of the resonant frequency set that each "spot" would have generated on its own. Depending on how long the drumsticks remain in contact with the drum head, you may well have a secondary damping of each others' frequencies due to the restricted movement at the points of contact.

That does depend on where the two sticks hit the drum head. At least mathematically, you could get a lot of different results by spacing radially and/or circumferentially.

Two sticks will usually produce a louder, thicker sound than one. So it emphasises beat 1 from bar 28 on. Just putting an accent played with one stick will give a subtly different effect. It will, of course, vary with where on the head the sticks are played - and there's no indication for that. And occasionally, the player may well not hit that beat exactly together with both sticks, (good or bad...), and maybe would be guided by the conductor.

All percussion instruments produce one sound from hitting the resonator and a different sound from the resonator vibrating. This is often overlooked because both sounds always occur together; however, using two sticks to hit the same drumhead simultaneously emphasizes one in comparison to the other. (This is precisely the kind of subtle instrumentation detail that Ravel is famous for.)