Advantage shouldn't stack - it doesn't according to 5E rules: even one source of disadvantage cancels N sources of advantage (and vice versa) ...may seem counterintuitive, but the rules sayeth thus:
"If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage."
This clarifying mrfuji3's point above for the rules lawyers out there