What happens if two upgrades tie?
The one that is currently at the highest upgrade will win. If the watchtower is at level 2 already, and the search tower hasn't been upgraded, and they tie, then the watchtower will be upgraded.
If they are both at the same level, then the one that received the tying vote first will win. If the watchtower got up to 20 votes, and then the search tower reached 20 after it, then the watchtower will be upgraded.