Since I’m leaving the game anyway, I will just say this…
Already in the second week of Guild Wars, my guild (Black Dragon) was propositioned by another guild to do exactly what was stated in the OP. He wanted both our guilds to set 1-troop defenses so that both our guilds could rise up the ladder. Our guild isn’t a “sister” guild at all with this guild. I was stunned that someone would think this was ok. But there it is.
You can bet this person isn’t the only one to think of it. You can bet some desperate guild will take such an offer to rise up in the future.
Frankly, I thought GW would be the one place you could have some competitive fun without cheating that goes ignored. Clearly, it isn’t.