Does mixing different 'potencies' with each other give you a better high? My friend is a fan of mixing dank shit with mids and it seems to work...

No it's more up to the strains that your smoking, either sativa or indica. It changes the high because it's different strains. Potency really has just a little to do with it, basically the amount you need to smoke to get really high. Mix a strong sativa with a strong indica will get you high as fuck and in a really awesome place.

I don't know if it's a "better" high, but it's a different high, a combination of the cannabinoids in each.

It's called a "salad bowl" :)

I've contemplating doing that with different weeds. I've gotten a strain that made you kinda drowsy, but more euphoric, and another that was more awake/alert, but I didn't think to mix them while I had both.