2 supplemental letters from two different people for Stanford

[skieurope]

I wonder how Stanford would view two supplemental letters from two different people.

Stanford is quite specific. Note that the emphasis is theirs:

You may submit one optional letter of recommendation if there is another person who knows you well and will provide new insights about you.

https://admission.stanford.edu/apply/freshman/evals.html

So, I would follow the instructions and send just one. Otherwise what they might think is the thicker the file, the thicker the kid.