The way AC works in D&D (and by proxy BG3) is you only ever use 1 formula for calculating AC. So if you multiclass you have ACCESS to both calculations, but only the one that gives you the highest AC will be active. SO, you can use armor, OR, do 10+dex+con for Barb, OR do 10+dex+wis for Monk, but they don't stack, and were never intended to stack.