Guild Wars 2

If we have 2 warriors and each one use the same banner, do the whole raid get the buff?

Does the game have any system that selects which players do not have the buff and then apply said buff if another banner is available?

no, you need to put the warriors in separate parties

That is not 100% true.

Banners, Empower Allies, Spotter, etc all apply to 5 targets. However, their reapplication interval is more frequent than the total number of time of the buff.

For example, the buff for a banner lasts 6 seconds, and the reapplication rate is 3 seconds. So if 5 different people happen to be able to swap closer and farther from the banners every 3 seconds, than in theory 10 people could have permanent buffs from 1 warrior.

More than likely, there will be some overlap between the first application and the second application.

This is why you see many guilds running one warrior in the first subgroup of a raid squad with 7 total people. In theory, they should be able to might cap, EA, and banners on 7 people instead of 5 just based on the randomness of the players positioning and the longer application duration versus the frequency.